Nothing prevented Ryan from running for the nomination except Ryan. Though I did not vote for Romney he got the most votes. Such is life.
Romney did not “just” get the most votes. He was hand-picked by the GOP establishment, who then went on to destroy his competitors while supporting him. They went after Cain and Bachmann, and then made sure that nobody could even oppose him in Virginia. Only he and Ron Paul made the Virginia ballot.
This was no fair contest that Romney won out of popularity with the people. He had no traction early. None. He won a war of attrition because the GOP establishment that hand-picked him then threw their full support behind him while trashing his opponents.
Bingo. Anyone could have run. There were lots of Tea Party types that could have thrown their hat into the right. They didn't. Ryan is a good choice for Romney. I wish it were Ryan at the top of the ticket, but he chose not to run in the primaries for the job.
Yes and no. The primaries have not conferred upon Romney as much legitimacy as his hard-core supporters like to imagine.
In the early primaries that Romney won - which were not many - he won a plurality only because the rest of the vote was being divided up between any number of other candidates. Romney only started consistently getting outright majorities after pretty much everyone else had dropped out, or when he was only facing Ron Paul.
So, to say that Romney won the majority of the primaries is somewhat disingenuous, since half of them were won without having any opposition remaining in the race.
Even then, Romney didn't do so hot. Here in NC, for instance, he still only won ~70%, despite being the only GOP candidate left. Even with that, 30% still said "no way."
Romney won because 6-7 conservatives split the vote in the early primaries and handed it to him. But that still doesn't give him real legitimacy as a nominee.