Because to import old content, you have to respect the old license (or get every contributor of back-then to relicense). That would mean having a site with contents under differing licenses depending on date, which is something the corpos can use as an excuse to continue siphoning everything without consequence.

I'm fine with a mirror / archive of SO. But it shoudl very definitively be a different thing than an active SO alternative, and their users and data storages should be also different.