I didn't see this before posting my longer response, but yes, plugin compatibility has a lot to do with it, though it may not be the only reason.

I don't entirely agree on the older hardware point though. I was unable to upgrade my laptop to Windows 8 because it didn't have a processor feature that Windows 8 requires. In terms of just general oomph, it could probably have run it because it benefited in performance from the upgrade to Windows 7 and Windows 8 is supposed to be a further improvement on that front. My older hardware (just pre-Vista vintage) has definitely had its support cut off!