Why? Because of how good he is taking down wrestlers (re: couldn't take down 240lb couture)? Or his proven knock out power (re: has terrible standup)?
He knocked out Couture with a less-then on the dot strike/half graze.
He made burger meat out of Mir's face with a 4 inch punch.
I'm not making the point that Brock has showed great KO power, but these examples certainly don't indicate that he's shooting blanks.
I don't think it'll be an easy fight but I see Carwin hurting Lesnar and then capitalizing it with a TKO, 1st or 2nd round.
I don't see Carwin winning, but I don't see the fight being easy by any means for Brock. I'm not counting Carwin out, but I'm definitely favoring Brock.
5X hands, whatever, Carwin was also a DII champ, whereas, Brock was a DI champ and a more dominant champ at that. However, your point that Nog could hold Couture down while Brock couldn't is a sound point. I hadn't really thought of that till you enlightened that point. It's definitely something worth thinking about.