Mello didn’t want to sacrifice himself for the Greater Good just so Near could win. He’d do it himself, even if it took him twice as long. Even if it meant hiding under the enemy’s wings in order to gain information. He would win.
(Or, in a much less cryptic manner: Mello doesn't give Near the distraction in the end, and doesn't die. Instead, he chooses to think for himself and join Light's side in order to defeat Kira from the inside.)