It's becuase in reality, his resume is extremely weak. He beat JMM, but it was very controversial, although it was close and John showed he could hang w/ a legit HOF type guy.
Other than that though, John just fights weak competition in Asia b/c he can make as much money there fighting bad fighters, as he could coming to the US and fighting dangerous opponents.
I like John a lot, he's a very skilled fighter and all that. But, besides the JMM win, his other two best wins are Rocky Juarez (beat him twice, but Juarez is basically like a journeyman), and Daud Yordan (a very solid fighter who I like, but nothing special).
I don't think John is a legend. He's a dude who had a lot of skills and talent, but never challenged himself enough to prove he was something more.
He fought Marquez, but there were plenty of really good fighters at 126lb (Caballero, Gamboa, JuanMa, Salido, Ponce de Leon, etc.) that he should've fought in order to show REALLY how good he was.