It could be due to the death of Spock's brother, the fact that the Enterprise-A was in pieces, and "God" that they found at the Galactic Core wan't used in any other productions, except for a book. It also didn't the Trek laws of physics. They were able to travel the several thousand light years to the galactic core in a day instead of years. Sybok was able to modify the engines to travel the vast distance and modify the shields to survive the Great Barrier. The Klingon Bird-of-Prey was able to scan them and do the same. They also were running out of movie and production time and needed to cut some planned events that would have been cool looking. The book was much better because it included Sybok's back story. Several special effects people were working on Star Trek: The Next Generation, or on other movies. There was also other reasons, and the list is in the movie's encyclopedia page on Memory Alpha.