Parties spend a great deal of time vetting their presidential candidates. Mitt Romney went through dozens of debates, hundreds of media interviews, and countless meetings with voters, donors, and activists -- in two election cycles -- before Republicans settled on him for their presidential nominee. It's a very public and expensive process. Conversely, just a handful of people (mainly Romney) were responsible for selecting Paul Ryan as the vice presidential nominee, and the decision was made entirely behind closed doors. Yet it's an important decision. No, the vice presidential candidate doesn't much affect whether the ticket will win or not, but Paul Ryan's influence on a Romney presidency would no doubt be considerable, and Ryan's own chances of becoming president someday just increased substantially. Do the parties have much of a say in vice presidential selections?
We don't entirely know the process by which Ryan was selected, but it seems likely that party forces weighed pretty heavily on Romney's decision. We have some evidence from 2008, for example, that John McCain wanted Joe Lieberman to be his running mate, but that McCain's advisers rejected this because the party would never accept it. That is, there was considerable fear that party activists would abandon the ticket if it had a pro-choice lifelong Democrat on it, and would either stay home or back a more conservative third-party ticket instead. So McCain made his famous game-changing pick. Not many members of the GOP were in the room when he did that, but the party's influence in limiting his choices was considerable.
Romney, like McCain, has been concerned about retaining the support of conservative activists given his own (recent) past with moderation, and he wanted a running mate who would remove some of the doubts about his ideological purity. Ryan has become the darling of fiscal conservatives and many in the Tea Party movement over the past few years, and tapping him sent a signal to those parts of the GOP that Romney takes their concerns very seriously and is willing to tie his fate to theirs. Romney may well have made this decision on his own, but partisan actors certainly played a very powerful role in constraining that decision.