They can update the production quality. The story is good; the special effects and costumes are hella dated and cheesy though. The TNG episode homage to it looks better.
I almost feel like this is rhetorical, but I'll answer anyway.
It's about money. As simple, and shitty as it sounds.
We see all these remakes and sequels happening because there is a built in fan base. It's guaranteed to make some profit, or at the very least break even.
An original movie made by an unknown writer and director is a huge gamble. Shit, even with well known ones. It can still be considered high risk.
So a studio has to make these types of movies, so they can fund the one original which may be a possible flop.
And yes, I am this interesting at parties if you were wondering.