User blog comment:Eustass el kid/New Nakama Hate?/@comment-4904935-20130425032342/@comment-972068-20130425045750

Because Jinbe was a perfect candidate. He was strong, for one thing, an ally of Luffy, he's outcasted from his home, wanted, and was even an ally before the timeskip. He was even in the cover of one of the volumes WITH the Straw Hats.

This is exactly why Luffy asked him to join! Sadly, he didn't refuse Jinbe's refusal.