There has been a 'why Paul Ryan' explosion on the internet media channels. Just about everyone is making the analysis too complicated. There are two essential types of Vice President picks: solidify your base voters or attempt to broaden your appeal to swing voters. A presidential campaign tends to choose which of those concepts they need to fill most. It was obvious that Romney needed a solidify the base selection, so names being bandied about like Chris Christie, George Pataki, or Jon Huntsman were non-starters no matter how much we might like to see rational Republicans get more exposure. So Romney had to walk a fine line between appealing to the base, without risking another Palinesque crazy-seeming person. Ryan was the only eligible choice in that regard. He usually comes across as non-crazy, he doesn't immediately look like a crazy person on television if you don't know anything about him, and his hyper Republican credentials are impeccable.
To people who have been paying attention the last two or three years, Ryan looks worse--an obstructionist on the economy, a tax-the-poor warrior, with an over-emphasis on budget and an under-emphasis on growth. But most people aren't political junkies, so given Romney's need for an appeal to the base, Ryan makes a lot of sense.
Interestingly, the last 'broaden your appeal to swing voters' picks look to be Joe Lieberman (2000) and Jack Kemp (1996)--both in elections where a solidify your base pick looks like it would have been better (presuming that a VP pick is all that important, a proposition that I find not all that convincing).