FNaF, when it was first released, had a lot of hype over it, which is understandable. I think that, for the time, there was no game that was like it. It was a fresh and extremely creepy take on the horror genre.
Then the lazyness hit. Not necessarily with Cawthon, but with the genre as a whole (or at least an oversized proportion of it). FNaF clones, most of the time, just took the game, removed the FNaF characters (usually the maps too), and replaced them with characters from other popular franchises, such as Spongebob, Minecraft, or Teletubbies.
Marketability is the main reason why the FNaF series hasn't died, and it's easy to see why. FNaF is an extremely well known series of games that had a reputation for being original and scary because of it (there's no way it can still have that reputation). Everyone's heard of it, many people have played, nobody wants more of it, but they'll buy it anyway because hype and marketing.
In short, FNaF was a great series of games that became worse as more of them were released (which is a far more common theme than you would think). It's like the Terminator series, in that it should have probably stopped after the second one, but it didn't because of hype and marketability. Let's face it, hype and marketability are the main reasons why there are so many crap products that are successful.
FNaF = Hype + Marketable = Success
CoD = Hype + Marketable + Lazyness = Success
Crappy music = Marketable = Success
Lots of mods from this site = Lazyness = Not a success (Apparently, most of us don't fall for it)
Beats by Dre = Hype + Marketable + Lazyness + Good for crappy music = Success
Music streaming services* = Marketable = Success
McDonalds = Marketable + Large quantitiies of sodium and/or sugar = Success
iPhones = Hype + Marketable = Success
My voice = Hype + Marketable + Sexyness = Success
* = That you have to pay a monthly subscription for
As we can see from these examples here, this problem runs rampant these days, and it clearly needs to be stopped... except for my voice, because I don't really know how that would work.