To be the conservative alternative to Mitt Romney, it helps if you're, you know, convincingly conservative*.
It's interesting to see that there is an undeniable de facto (if not intentional) cooperation between Mitt Romney and Ron Paul. But why? I can only speculate but... If Mitt Romney gets the nomination, Rand Paul would be a brilliant choice for running mate.
As a side note, if I ever run for office, I want to hire Ron Paul's production company. His ads are just great.
*I know I'll get grief from my Santorophile friends but before you send hate mail, watch this cringe-worthy ad by Rick Santorum himself boasting of working with Barbara Boxer and Hillary Clinton. Don't let him spin that he was trounced in 2006 because he heroically held fast to conservative principles.