First of all, because they added some unnecessary characters, like Susie's (Amy Poehler) British and unfunny boyfriend. A new yuppie couple (Mark and Vivian) who is very intimate with J.J. (Zak Orth), and Coop's (Michael Showalter) fianceé. They were a nuisance, and I felt like their scenes were just not interesting or relevant to the whole plot.
I found some contradictions, because in the original film, McKinley and Ben are very closeted about their relationship, and they were very open in these episodes. Let's remember that this was the height of the Aids scare (before the movies like "Philadelphia" helped mainstream people to sympathize with HIV positive gays), and being openly gay was not so accepted at the time (especially in a place with children).
Susie and Ben were very close (they ran the drama classes) while they barely interact this time.
What was missing this time, were the children themselves. Despite one teenager alpha male (Deegs), there were no child humor like we saw in the movie and series, which used to make me laugh a lot.
There were some plot holes like what happened to Gail's former partner (in the end of the movie, she gets engaged to a 11 year old kid), what happened to the robot guy (who in the movie was J.J. brother), and in the end, there is a scene on the beach that I won't explain because there are spoilers.
I had a good time watching it, even though I think that I was hoping that Coop's love story would have ended differently.
The best of all for me is the bromance between Victor and Neil. The last being the professional sidekick of a person who is actually below him.
Watch, but keep expectations low.