Peeves isn't in the movies for the same reason Hermione's side plot in the fifth book about house elves isn't in the movies; it is not important to the main plot and the books are long enough without adding additional characters.
I was a little sad about that. At times he was hilarious. It probably all comes down to money. like they said, he wasn't a Main character with an actual plot and with everything else that they put into the movie, it wasn't a necessarity.
The producers said it would cost too much to include peeves in the movie,also the could not find a perfect character to play the part,Witch its sad cause i would have loved to see him in all the films.
i missed peeves in the movies ;n;
i guess like people said, he would take a lot of extra effects and extra lines and they had to cut stuff already... they left out Charlie weasley too..i hate when they just blatantly leave out characters completely..