User blog comment:Xilinoc/Ch. 564 (Mask is too fabulous to live)/@comment-117598-20140115180531/@comment-5398287-20140116015357

Well, if the filler Zanpakuto arc is anything to go by - and in this case, it pretty much is, since it's the only time this was mentioned - Zabimaru was dissatisfied with Renji because he had lost sight of what he was trying to achieve. This seems to be why he wouldn't lend Renji his full power. However, Renji has one hell of a purpose and drive now - kick the Wandenreich's collective ass. Additionally, he's grown much stronger through the training, and his lack of strength was also mentioned in Zabimaru's rant, so in all likelihood that's why Zabimaru finally decided he was worthy.

Just my two cents.