Well, that's a good question, and I actually thought he might go Rubio when he got the nomination, I've said here for months, that Latino vote, record breaking this time around in terms of both just how many Latinos voted and what percentage of the electorate they made up, REALLY bolstered Obama's cause and clinched some key states for him.
Rubio is a divisive figure in the Latino community, at least among Latinos I know here in CA, some like him, some don't...he's Cuban-American, and that's different from Mexican-American or Salvadorean-American and so on, and the Florida Latino electorate is different than the electorate of Latinos here on the West Coast near Los Angeles, but STILL...would've helped his diversity, among minorities, Romney was slaughtered.
As close as Florida is, if he'd gone Rubio, he almost certainly would have taken that state; granted, as Obama won by 100 Electoral Votes, 29 wouldn't have given him a win, but it just shows the difference Rubio might have made.
I THINK The reason they chose Ryan was to appeal to the far-right of the party' there were questions initially if Romney was conservative enough and, a big issue among many Republican voters in the far-right, his religion; his Mormonism was initially off-putting, so perhaps a Mormon and a Latino might have been too off-putting and progressive to a rather seedy side of the Republican electorate (ESPECIALLY Tea Party voters) and thus we got Ryan.
Also, the picked him to try and challenge Wisconsin and expand the electoral map and give a greater chance at victory in that state which they still lost...
His youth, to try and appeal to younger voters and a younger generation....
His Randian politics...
It was a failed choice, but I think that was the thinking behind the choice.