I've heard Sherk say a million times how he's been boxing for a long time, even in the post fight interview he was saying, "A lot of people don't know I can box, but I can box" and it really isn't true. He might be able to shadow box, but he was obviously so perplexed by BJ's simple head movement, jab and 1-2 that he had no way to counter it.
In a lot of MMA fights, guys can get away with throwing the same combos or counters over and over because the fight changes so much that there's not enough time to adapt what you're doing but this wasn't the case in this fight. Penn never changed what he was doing because it was working perfectly, Sherk needed to change what he was doing.
It's not like this is the first time either, Sherk did the exact same thing vs GSP as well as Diaz. He did beat Diaz, but I saw that same left hook come up short over and over again but Diaz wasn't able to capitalize the way Penn or GSP did. Sherk just seems content to throw the same combos that don't land, I have no idea why. I think he's taking his boxing as far as it can go with his current trainers/sparring partners, it wouldn't be a bad idea for him to find a boxing gym to supplement his current training because it's obvious that his takedowns/top game are top notch already. Improved boxing could have potentially won this fight for Sherk, though I think a 100% Penn is the closest to unbeatable as any MMA fighter can be.