Because the moviegoing audience wanted 'splosions. They wanted conflict. Not people arguing on the bridge and looking out the viewscreen for ten minutes. They wanted a blockbuster. Well, the Trek movies didn't really reach that point more than a handful of times, but damnit, they tried.
WOK wasn't a success because of action and explosions, it was a success because it stayed true to and elaborated upon the characters we knew from the show, because it was well paced with a good story, and a plethora of resonant themes that grew organically from the plot, and well placed action that furthered the story when it needed it.
Not that most casual movie go-er can articulate such things, but I'd argue TMP skates by more on it's level of visceral experience way more than WOK does. One need look at the new Trek movies, Transformers, and similar schlock to see movies that are a success more for the explosion a minute set pieces and editing than for a story and characters.
The point I was trying to make (and maybe I didn't convey it well enough... I'm tired) was that the 80s was the beginning of a turning point in film from smart features into blockbuster schlock. Particularly in the 80s, you see that with the Trek movies.
TMP is very cerebral. TWOK and TSFS are fairly good romps, with TSFS a little less so. TVH is a comedy and while it has a nice message, I don't think either hold up today. It's too over the top in places. TFF...Granted, it was mired by a lot of problems with budget and the writers strike. And really? Even though it tries and the Kirk/Spock/McCoy stuff is great, it just isn't good overall. The forced comedy. The action sequences. Ugh. And personally, I like TUC. It, however, can be argued that the film destroyed characters in order to make its point and have a lot of action. But by the time you get to the TNG movies? Well, the less said the better. By the time you get to Nemesis, it truly is all just 'splosions and truly destroying the characters to make its point. And as entertaining as I find them and as much as I enjoy them, yeah, the same thing could be said about JJ Trek.