Nick has hyped his fights up better and has had better fights overall and has fought the better guys on a constant basis or whatever.
Look at GSP/Nick, that fight sold so much not because of GSP, but because of Nick. People probably won't believe it but isn't it one of the biggest PPV's they ever had? if Nick didn't talk the way he did or get a rise out of GSP the way he did, it wouldn't have sold as much. Nick is just as much the reason that sold so much as GSP is.
Nate has never hyped a fight like that, never put on a fight that big before and probably never will because there's just no GSP-like stars around, the guy is basically one of a kind and Nick got the chance to make that shit happen with him.
Nate is losing to guys like RDA (wasn't even close to 100% in the fight) and Nick is fighting, and probably losing to GOAT's like GSP and Anderson Silva. If Nick wasn't getting those fights, he wouldn't be as much of a 'needle mover' either.
Nick seems to know how to work the system a little better than Nate. He knows how to make fights bigger and hype them up better than Nate does. The hype for Nate/Cerrone is probably the best in terms of hype for Nate, in my opinion, and man, that was a great fight for us Diaz fans! if Nate could do more of that, he'd definitely be a 'needle mover'.
And all in all, sometimes it's just that one guy will always do better than the other and there's no great reason for it. There's almost always the one brother who will be a bigger star than the other. Like Fedor/Alex, Fedor is the GOAT, obviously, but Alex was a great fighter and all but he'll be just known as, "Fedor's brother, the guy who got KTFO by Cro Cop and is being charged with rape", even though that's not the best definition of the guy. That's just how it is, it seems.