Scott Cawthon, the developer of the Five Nights at Freddy's games announced in his Steam news post that he will put the next game of the franchise on hold. The reasons behind this decision is simply because the developer doesn't feel like working on it at this moment.
"After forcing myself to keep working on it day after day, I realised something - I just don't want to work on this,"
After every finished game of Five Nights at Freddy's the developer feels like he has to put the bars higher for the next game, which gave him a lot of pressure and so unable to continue. It is quite possible that the developer needs some time to refresh his creativity. In the meanwhile Scott Cawthon stated that this does not mean that he will take a break from making games as he said that he will focus his effort on other project which are fun.
Although this is not the first time that Scott Cawthon "cancelled" a game, back in 2015 he posted on Steam that Five Nights at Freddy's 3 was hacked and therefor cancelled. But later it was discovered that it was a joke as the files in the leaked game was just his previous game "There is No Pause Button".