I'd also argue that a webcomic can support a larger number of plot twists and characters than a movie simply because it has a lot more time to work in. Most of the long-running webcomics I've read would take far more than a single movie to cover, and would be better compared to movie series or TV shows.

(Granted, you also need to allow that extra time means that only the devoted fans will remember hints you dropped months or years ago... that may not be a problem in Homestuck fandom.)

Basically: different media tell stories differently.