I'm almost certain the actual reason is producing all the infrastructure to DMCA content on Google/Facebook/Twitter/Youtube and other "mainstream" Web 2.0 platforms is expensive.
Copyright enforcement is a lot stricter on Youtube, than say, Reddit video.
It's all down to the engagement of the IP owners.
My guess is they just haven't gotten around to chasing Archive.is down that hard.
It’s a value proposition. Archive.is doesn’t host Hollywood blockbusters. If it is hard to take them down, nobody is going to do that as long as they don’t cause to much real damage.
