Other than the fact TMNT and X-Men were basically made to be franchises, they didn't exactly make sequels out of good films this year. Huntsman 2 was unnecessary and made purely out of spite to Kristen Stewart, comedy sequels are either all in or nothing, Zoolander 2 was released opposite a movie that wasn't that much of a surprise hit, and Alice 2 really didn't need to be. And opening opposite X-Men was just passive aggressive. Seriously Disney... you ain't never getting X-Men back from Fox.
TMNT fans probably didn't have patience for that continuity continuing, despite being a better TMNT movie. And X-Men Apocalypse got bad word of mouth, especially since the well received 3 movies before it (including Deadpool which outgrossed the entire series). But this isn't an exclusive to this year. Kids refused to go back to Smurfs a second time (then again, it opened opposite like 3 or 4 kids movies when the first one didn't), MMW didn't get the love or attention that TM did. Ted 2 was treated like crap for some reason (to be fair, yeah it wasn't quite as good as the first one, but they had a logical follow up that wasn't a retread of the first one). And on the other hand, it's not like this year's movies were exclusively sequel avoidance. Unless it had a comic book character from Marvel or DC, or was a Disney cartoon, no one really flocked to anything this year. I don't see why "sequelitis" is to play here when I'm guessing no one thought any of these were important enough to actually see in theaters, rather than wait 2 and a half years to put it on their Netflix queue and never get around to/watch to kill time on the train to work on a phone like the directors intended.
That said...we don't *^&%%$ need a Pitch Perfect 3!