Why? Well, the last couple of times LJ stuck their head up their ass with a bad PR move, all the people doing the "We'll just go somewhere else!" seemed to run to InsaneJournal. And now IJ has proven to be, shall we say, less than stable. Hosting provider issue or not, in the end the site is down and they came close to losing a month of data.
And this is history repeating itself - with past exoduses the bulk of people fled to GreatestJournal. And then GJ imploded under the strain.
I've said it before, and I'll probably say it again, running an infrastructure that can support the kind of load LJ does and making it (relatively) stable is not easy. You can't just download the LJ source, setup a couple of servers, and welcome all comers.
I'd still rather trust my data to LJ than one of the upstart sites trying to run the LJ code. At least LJ seems to be *technically* competent, even if they seem hell-bent on attempting to perfect a recursive rectal-cranial inversion on the PR front.