Well, focusing on the main GC guys, I'd say it's not just a matter of this year Giro. The thing is: it's extremely hard to be solid contenders in two GTs in the same year.
Think about that. How many riders have concretely tried to win two GTs in the same season in the last ten years? Or an easier research: how many of them have podiumed in two GTs in the same year? We're not talking Contador here, we're talking human beings. Nibali and Sastre (who actually almost accomplished the task three times, amazingly so).
It's hard, guys. It's actually very unlikely to happen. You need to be one of the best around. Especially in contemporary cycling, which is (I presume) cleaner than before.
One could argue Nibbles did that last year, hence proving this year Giro was the only thing that prevented him from doing it again. But it's not as simple. Liquigas had scheduled a completely different season for him last year. He went to the Giro as a superdom, with no pressure at all, he wasn't even very prepared for it. Liquigas controlled the whole race amazinlgy, some stages were perfectly suited for him (Asolo) and nothing happened for the first two weeks basically giving him time to get good legs.
Then he went to the Vuelta and found one of the weakest competition in years.
He was lucky. An amazing rider, if you ask me, one of the smartest around and a true fighter. But you gotta admit that, he was lucky.
I've been lurking this forum for quite some time now, and I've read surprised comments on Scarponi being dropped, or Purito fading, and on and on. "How come they didn't prepare well for this Vuelta?" They prepared. They really did (I'll keep Anton out of this discussion, I don't know about him). They just don't have enough legs to compete and it's completely understandable after a whoole year at the top (all of them were already being strong in April, even March, go and check).
Had the Giro been less brutal, we might have seen more fire from these guys, but it's not safe to assume they would have been able to win the entire race otherwise.