Perhaps because although capacity is strongly influenced by inheritable traits, to actually be at the point where your intelligence makes a difference you need to have put considerable effort into it as well?
Also, in the main, intelligence and education aren't necessarily the same thing: education itself rarely gets you anywhere, whereas your education does. There's plenty of people who will criticise the tendency of public school old-boys to pick public-school old-boys for plum roles in government and the civil-service - that's education, but it's no real judgment on their intelligence.
O.