Cowboy has had a great career, but he's simply not a championship level fighter. He had 3 cracks at a WEC title (2 against Benson, 1 against Varner) and 1 crack at a UFC title (RDA), but he lost all 4 fights and was finished in 2 of them.
He's always been just a notch below championship caliber, which is fine. Like I said, the guy has had a great career.