Cain took him down after he hurt him very badly standing, I am not sure he would get this opportunity against Frank. Also Frank has made a point of working on his wrestling since the second Brock fight. Cain is not a high calibre wrestler, his wrestling is very good but he doesn not have amazing credentials.
He actually took him down before he did any thing to Lesnar standing. It's the other way round, watch it again.
Cain is a top 3 MMA wrestler, up there with Bones and GSP. People don't seem to realise just how damn good his MMA wrestling really is yet. He can take any one in the division down with ease and beat the hell out of them.
Mir's wrestling still sucks too. He couldn't get a single take down on CroCop.
Neither Jon Jones or GSP have amazing credentials either, yet they are the top dogs when it comes to MMA wrestling.