I think it's because he had a solid game lol. The only good thing he did was that lovely chip for James' try. He also made a few mistakes, another kick out on the full (how many times till he learnes?), and he was suspect in defense on a couple of occasions. Did he really get man of the match in front of the likes of Martyn Williams?
I think he got MOTM becuase his tactical kicking showed a significant improvement, especially towards the second half. The territorial kicks kept the ospreys in their own half and kept the pressure on. I agree that Martyn may have been the stronger contender though, although he'd probably get it every game on that basis .
I think people suggested SNK had a good game becuase he hasn't been as bad as he has been. Give him a bit longer and I think he could be quite a decent little signing.