Because Romney's campaign advisor "gaffed" (definition of "gaffe" in D.C. supposedly being when a politician accidentally tells the truth) and made the Romney etch a sketch campaign strategy public, Romney could not be blatantly obvious about it.
True, all Presidential candidates, maybe all candidates these days, do run to the left or right in primaries, then veer center during the general election campaign, but Romney's guy was the only one I know of (so far) who was stupid/naive enough to say so--and in such a memorable way.
Romney failed at winning Kennedy's Senate seat. He also did a lousy job at being Governor of Massachusetts. Sratch that. He never even tried to be a good Governor of Massachusetts.
So, he really had nothing to point to, other than his success in the private sector--no small thing, given the current economy. However, that got tainted by Kennedy's ads in 1994 and, more recently in the Republican primaries and now by Obama's ads. That pretty much left Romney with nothing untainted except his success auditing one lil ole Olympics in Utah, now known as "the middle of nowhere." And then, Romney opened his yap, leaving Romney pretty much with nothing.
And, until now, his strategy has been to run simply as "not Obama." That has not worked, as the steadily widening gap between Obama and Romney in the polls indicates.
Enter Ryan, who will embody an entirely new strategy. He will be Romney's attack dog. He will say things that don't 100% mesh with things Romney has said during the campaign to date and so on.
See, when you're Romney, you are not limited to plastic toys. You get to buy a toy that, while maybe lacking a heart, a conscience or scruples, at least looks, walks, talks and breathes like a human.
Come to think of it, there's ample precedent for that.
http://www4.pictures.zimbio.com/gi/Dick+Cheney+Turns+70+Y5L-4uSftgfm.jpg