Show was tremendous in every thing he did on this RAW. His promos, believability, perfect work. He SAVED this show two weeks in a row now from "worst of all time" material. His promo on Clay, then the spear and the beatdown was a heel from another era, great booking from WWE for once but amazing execution from Show. Glad that he can show what an awesome performer he is.
As for the position, Big Show is the top heel and a main eventer since the day he got into the business, of course he's closing the show.
Mark Henry plays the "unstoppable monster" character far more convincingly than Show. I'm not saying Show is bad I'm just saying my expectations for that type of character are so high after the incredible work Mark did last year.
Anyway, the answer is sadly WWE see Punk and the WWE Championship as a secondary item, they see Big Show/Cena as their money feud not Bryan/Punk so that will always get the main event spots, even when Cena's not even there.