He put work into tanking it. He earned so much goodwill after FNAF 2 came out. So many people were impressed with the speed at which it came out after he announced it, the game was well recieved, he made YouTube careers for theorizers and LPers. He had an entire cottage industry sprout up around his game out of pure enthusiasm.
Then he started telling his eager fanbase that he was done. There would be no FNAF 3. And then there was. And then he was saying that he was done with the series again. There would be no FNAF 4 because he didn't want to do FNAF anymore. And then he did. It was between FNAF 3 and 4 that he really tanked the series. His community was getting visibly upset with him for jerking their chains all over. He wasn't interested in the series anymore and wanted it done, and why should anyone waste their investment on an IP that even the creator doesn't care about? But then he kept making games after 4. After publicly quitting the series three times. The goodwill that he had in the beginning isn't there anymore, it just looks like he's flogging the series now. He still has a fan base, but it isn't what it used to be. And he threw away his chance to make FNAF a real powerhouse.
If he had stuck to it and managed his brand, he wouldn't be competing with Undertale for Internet niche points now. He had a marketable IP with a lot of enthusiasm behind it. He could have had a NetFlix series and a Sony Pictures movie that made $400 million at the global box office by now, if he had pushed it instead of trying to kill the series after every game.
FNAF was what most creatives go their whole lives failing to create. An IP that had the potential to reach general audiences and join pop culture.