He beat the current WW champ, when he was a huge underdog. (still the only man to ko Lawler iirc)
He beat the #1lw in the world in Gomi, when he was a huge underdog. (with the rarely seen gogo)
He beat the absolute shit out of BJ Penn, a 2 division champ/legend. Diaz retired a legend in Penn.
He beat the absolute shit out of Frank Shamrock, former UFC & SF champ/legend, at a higher weight than Nick was used to. Diaz retired another legend in Shamrock.
He has a good chance imo to beat Anderson Silva in a couple weeks (we'll see). He may retire his THIRD legend.
He's also beaten Paul Daley, the previous #1 or #2 UFC WW contender.
He beat (imo) Carlos Condit, the #1 UFC WW contender.
He's just never gotten the UFC gold. Which is part of what makes this issue a little tricky. He's also one of those guys that you can't just look at fightfinder and understand their career. Most of us that were there watching him along the way, understand how damn good that kid really was, and how many times he's shocked the entire sport with his upset wins.