I suspect that, quite apart from time constraints, Bacchus got left out due to worries that audiences wouldn't know who he is. I suppose there might also be some concerns that his presence muddies the Christian message enough to make conservative Christians boycott the movie.

Prince Caspian opens up the world to include the pagan by showing the natural world as closer to God and having it break free of constraints as part of rebalancing everything. I doubt Lewis saw it that way exactly, but it's there.