Because Scott Walker wasn't even close to intelligent enough and Rubio is too scripted, inexperienced, and empty of any original ideas to become POTUS. Say what you will about Trump, Cruz, Bush, Kasich, and Christie, but they can think on their feet or at least have a depth of policy knowledge that wasn't just drilled into their head by some donor.
I'm not sure about Jeb! re: thinking on his feet. He'd just stutter and look awkwardly at the camera if he got into an exchange like that rather than repeat his talking points endlessly.
