Women, more than anything, are valued on their looks. You should know that, bad example.
What I'm saying is that it's just a question of time. The more time Nole spends at nº1 and winning slams, the more endorsements he will get. It's just a matter of popularity. And that he is not from a big country also hurts him, undoubtedly.
And you think looks don't matter for men? Why do you think Beckham is the most valuable footballer in the world endorsements wise? Certainly not his Messi or Ronaldo like skills.... Someone like Stepanek would never be a big endorsement guy even if he won 20 slams - those are facts sadly. I've not said Novak won't get more endorsements - I'm just saying he won't exceed Nadal and Federer. He got to no.1 in 2011 - he's had coming up to 2 years of time at the very top - so I don't think more slams will change much really. There are intangibles that matter and he can't change the fact that he's from Serbia and has a difficult name. Azarenka was no.1 all of last year, is not bad to look at and still has very few endorsements.
You need the whole package - talent; looks of some kind - be it classy like Fed or more boyish like Nadal; epic matches to get into public consciousness (Sharapova won Wimbledon at 17 - more memorable than Azarenka winning Aus Open at 22); rivalries; background from well known country with a good image etc. (Note that Sharapova seems practically American most of the time! Most people don't even know where Belarus, where Vika is from, is on a map.)
All in all, Novak is not going to be crying about money anytime soon. But in his career he will not earn more from endorsements than Nadal and Federer, no matter what success he has in the future. He'll be 26 this year - again hardly like he's a new bright young prospect.