Plus Takei was at that point really starting to push for his Sulu show, so he likely figured less screen time in exchange for the chance of a leading role in a TV series was a fair exchange.
Though he's not as underused as, say, Crusher, I always found the relative lack of Worf in three of the four films a bit odd. First Contact
is the only one to give him something that isn't either bad comedy or looking grumpy, yet he was only behind Picard and Date in the recognisability and popularity with the more general TNG watchers, so you'd almost expect a good sized piece of the action for him in each film.
is especially odd on that score, the opening 24th century scene that's specifically intended to introduce the characters, their relationships and overal role in the grand scheme of things to film audiences is all about his promotion and makes it look as if he is indeed a major character for the forthcoming events.
He then proceeds to do nothing except growl "Plasma coil" and "Romulan" every now and then for the rest of the running time. Despite the fact the secondary villains are his personal arch nemeses (something you wouldn't even guess from just watching the film).