anyone who wants to try and run is free to do so.
but electability in the general election is a factor alot of republicans are interested in. they don't want to send up some litmus test pure candidate, who can't win. the base can't elect anyone on their own, that doesn't mean they aren't important - but to the exclusion of sufficient independents, the base can't win it on their own. they couldn't save Talent and Allen and Santorum in 2006.
the party has done a poor job developing conservative candidates, who can bring a convincing message and communication skills to sway voters. I wish it were different, but it isn't.