Well, one problem is that you have a lot of self-described conservatives who are angry at virtually every possible VP alternative. For some folks, any Republican who has said he/she supports Romney is a "traitor" -- which knocks out everyone from Palin, to Rubio, to Ryan.
But I agree with your point -- Romney must make a real effort to appeal to conservatives, because we are the core of the party. I'd like to hear him give a bunch of those classic Reagan lines at the convention, and to nominate as a running mate someone who can articulate that message strongly.
The core theme of this election -- at least to me -- is for the American people to decide whether the government is the solution, or the problem. For Republicans to win, they have to be able to make that argument convincingly. And that will take someone as VP who can hammer that issue.