I'll go with the simple fact that audience love to pity and sympathize with a character: especially if the guy in question is incredibly good-looking and has a "romantic" relationship with his younger best friend to a certain large part of the fandom. Axel wasn't one in CoM, but once KH2 hit, he became one of the ever growing Woobies that the series is becoming fond of, as killer kung fu bitch so discreetly explained. Axel himself has the "bad boy" charm going on.
It doesn't help that he's the only member other than the main characters that Nomura ever gave a rat's ass about to go in-depth with because of his blatant bias and favoritism being chosen over neutral and fair development of others.
That's how a character makes it in the KH universe: get the favor of the staff, or fans, and you get whored out unnecessarily until you're dry...and even then some.