The same reasons I'd give up on any other story in any other medium. Could be that the story bores me, could be that it's alright, but not enough that I remember to check it for updates and it just gets forgotten. I think webcomics in particular are prone to pacing problems, though. A lot of things that read very well when binged all at once don't work out so well when you're only seeing a page a week. But you also can't just rush everything, that causes its own problems.

I'm no expert story teller, all I can say is the best webcomics I've read made frequent use of humor, mini-wham moments, and small revelations about mysteries that have been set up within the worldbuilding or characters, so that each update has something going for it. Just so that when I check the latest update, I don't just go "ah, filler panels again this week. Drat." I recognize just how difficult that is, though.