You're on the right track. These days to win almost any major race a guy has to be on top form and time his peak perfectly. There are plenty of guys who won't get to ride the TDF who will focus 100% on the Giro. If anybody wants to beat those guys he'll either have to be that much better than them or he'll have to be on top form himself. I think Contador is the only guy who could win without being 100%, and I'm sure many would argue that even he couldn't do it.

Same goes for the TDF- lots of guys want it and build their whole season around it. The Giro and the TDF are too close together for a guy to recover and build back up to his peak in time (plenty of time to recover, but not enough to build back up). It doesn't help that the Giro is usually insanely difficult with tons of climbing. So if a guy wants to do both races, chances are he won't be in top form for both of them. It's hard to do well when not in top form.

Nobody was able to be top 10 in both races last year. Sastre and Vino both did top 20, but neither was ever a threat to win either race.