Linux (Linus T himself iirc) is very GPLv2.

They are not onboard with the innovation that was GPLv3. That innovation was mostly about software patents iirc. But the result was also that big corps liked GPLv3 even less than GPLv2.

On top of that it was going to be very hard (near impossible) to get all devs to agree to a v2->v3 license bump.