Why did it take so long for people to discover the demon crown?

So in the futuristic year of (whenever Cave Story takes place) where seamless teleportation, regenerative rocket fuel, androids with free will that don't immediately dick everything that's around them up, and (what i'm assuming) translation devices that can work almost immediately, everyone and their mother tries to invade Mimiga island to get a hold of the Demon Crown.
But wait a second, why did it take this long to do so? It's a goddamn island floating in the sky, people would be trying to see what's up there the week reliable aviation was invented. The only sentient species of the place are Mimiga and Gaudi, the ladder of which is tightly locked away from everything else. People would have colonies up there before the year 2000, then after a few months they'd find out about the Demon Crown. The only threats i can think of to colonists would be Jenka, Misery, Drolls, rabid Mimigas, those goddamn sperm cats, and that's about it (I'm assuming mechanical looking creatures like Balrog and those labyrinth fan things were invented by humans, so if i'm wrong about that, please let me know), as everything else can be killed with a few shots from a basic pistol.
So did Jenka keep the island invisible for hundreds of years? Were people so fucking stupid that they just didn't see anything strange about an island floating in the sky? Are floating islands just a natural occurrence in this world? I'd love to hear what you guys have in mind

 

The Island floats very VERY high. This should mean what atmospheric pressure on this island is very low.
People were afraid they would just choke there due to lack of oxygen.
That is the reason why they send the robots to investigate first.
Then robots have discovered availability of oxygen, humans began to go there aswell.







What the heck did I just write?

 

My take:
When the countries of the earth sent their forces to the island previously, a war broke out that turned ally against ally. The human's killer robots were powerless against a biological threat that turned the world's animals against them. And Mimigas, an intelligent explosively powerful threat that quickly multiplied and regenerated; blinded with rage towards its targets, shrugged off modern weaponry. (Not to mention the appearance of those who used magic to send Mimigas into critical locations...)

At the end of the war, with the destruction of the Demon Crown, the island became off-limits. A no fly zone. The story of the island known to the world's population, including how its native inhabitants were a victim to the greed of those who came seeking its power. A political catastrophe. And the island, a former wonder of the world left in ruins.

Unapproved visitors to the island would probably be automatically dealt with. Not to mention the island continues to move unpredictably through the clouds, making it a treacherous moving mountain in the sky.

TL;DR Demon Crown was destroyed. OR should have been. It was also appears to have been buried until the Doctor found it.
Think of it like the ruins of Jurassic Park. You aren't allowed to fly there. And if you do, you probably won't survive. Not everyone has military weapons, helicopters and a body that can shrug off what killer robots could not.

 

Here is a little justification for my theories:

My take:
When the countries of the earth sent their forces to the island previously, a war broke out that turned ally against ally.

-Curly says the countries of the Earth were sent to get the power of the island. Until one man got the Demon Crown and(?) Robots control of the Core and the island, and with the robot's mission a success the island fell silent...

The human's killer robots were powerless against a biological threat that turned the world's animals against them. And Mimigas, an intelligent explosively powerful threat that quickly multiplied and regenerated; blinded with rage towards its targets, shrugged off modern weaponry. (Not to mention the appearance of those who used magic to send Mimigas into critical locations...)

-As told by Jenka, the Killer Robots hunted down the Mimigas. It seems likely they had knowledge or were protecting the Demon Crown or the Core's secrets. Why else would the Killer Robots push the Mimigas into a corner? Once the Mimigas ingested the Red Flower the tragedy began. Besides having the power to fight back, they were under the control of the Crown Bearer, Miakid. And he had control of Misery and all her magic and the Core. Also, I posit that all the creatures on the island which Quote has to fight, are normally not dangerous, until the Crown is taken. Mimigas with limited fighting skill seemed to live on the island, full of creatures that can defeat Quote. I would suggest Ballos' Rage effect applies to less intelligent species as well. (The theory how Red Flowers and the Core are connected to Ballos' power, and have subtle influence over the island.) And I suggest, with labs and bases that host Sky Dragons and the Core. Facilities were setup that could come under Miakid's control to facilitate an actual war.

At the end of the war, with the destruction of the Demon Crown, the island became off-limits. A no fly zone. The story of the island known to the world's population, including how its native inhabitants were a victim to the greed of those who came seeking its power. A political catastrophe. And the island, a former wonder of the world left in ruins.

-And back to the question you had, why was the island left alone? A place so dangerous that it caused a war for its power? Either it disappeared, or more likely... It is off-limits. Plus, the Demon Crown was destroyed. Though, little do they know that it regenerates as long as Ballos lives.

Unapproved visitors to the island would probably be automatically dealt with. Not to mention the island continues to move unpredictably through the clouds, making it a treacherous moving mountain in the sky.

-The background cloud movement suggests the island can move pretty swiftly. If there are no airstrips, and the island is high enough, and factoring in the possibility of turbulence... Not to mention despite having Killer Robot technology, the Sakamotos came in a pretty regular looking helicopter... Perhaps aeronautics technology isn't as high tech as other fields? And instead arriving on the island requires the right circumstances. Oh, and if the side of the island is any indication, it might be dangerous to approach. Hoppy things, ghostly things, sky dragons at one time??

TL;DR Demon Crown was destroyed. OR should have been. It was also appears to have been buried until the Doctor found it.
Think of it like the ruins of Jurassic Park. You aren't allowed to fly there. And if you do, you probably won't survive. Not everyone has military weapons, helicopters and a body that can shrug off what killer robots could not.

-The scene where the Doctor finds the Demon Crown. It's pitch black in the intro. Perhaps that area was caved in? It would make more sense that the Demon Crown could not be found readily, if it wasn't just sitting out in the open. The Doctor weaseled his way into their group, with secret prior knowledge to related subjects. Perhaps that's why he was able to uncover it? Or worse yet. Perhaps it called to him, like the cursed item that it is. Misery had it created by Ballos, and it cursed her into its service. I doubt that is what she wanted.

Or maybe zxin is correct, and floating islands/magic exist as a common thing in their world. That might make tracking a particular flying island difficult. And maybe the island stayed away from populated areas. The place where the island falls, certainly looked wild. Or magic kept it hidden.
In the end it's left up to the imagination, and what little facts Cave Story gives us to work with.