I'm just going by what I read. But maybe it's the case, as seems to be with Danny Kent so far, that they are good Moto3 riders but not as good as people such as Rins on the bigger bikes.
Look at Stoner. He did well in 250 but didn't set the world alight, who knew he'd be the only person who could ride an 800c Ducati.
Which takes us back to the original question, is Moto2 the proper breeding ground for graduation into GP. Its entirely possible that a rider could suck in Moto2 and excel at GP, but will never get that shot.
