Have you seen the kind of roles Kevin Nash gets? Have you seen Dead or Alive?
Marvel Studios will not cast an old, broken down wrestler as Thor. They won't cast a pro-wrestler as Thor period.
I love Samoa Joe. And not to say that he could never play a movie role. But he ain't playing Thor.
For all the "success" the Rock has had. He's not getting the huge, career master thespian defining roles. He's not the new Schwarzenegger, and the studios won't cast him as a super hero or in a huge budget tentpole like this.
Also about the height. It really doesn't matter. You guys nitpick about height so much, but in the end on a 40 foot movie screen it won't matter. Hollywood for years has made people forget that Tom Cruise is shorter than most of his leading ladies, and shorter than the average man. The short hobbits in Lord of the Rings were played by regular sized actors.