[jseliger]

Probably because they don't have the budgets and time to do it. This kind of stuff requires time, money, and energy to coordinate submissions and peer review and so on. I've only seen this from the humanities side of thing, and especially the journal Post45 (http://www.post45.org if you're curious); there are a lot of hidden, niggling issues that make creating this sort of thing harder than it looks.

And that's ignoring the already-established reputational effects that current journals have.