Might be because no matter how many more wins Chimchar gets Ash will favor Buizel more. Every Trainer plays favorites. It's just that their favorite is not always necessarily their strongest Pokemon. You can kind of feel that there's a strong bond between Ash and Buizel. In comparison Chimchar is usually let out of the ball to do its job and then put back in the ball. Sure Ash treats Chimchar better then Paul in the sense that he doesn't make it battle until it keels over but is there really a connection between the two?
To be honest, I disagree. At the time it seemed like Ash was favouring Buizel over everyone, but then after Fantina's battle, Chimchar kind of took the shine away, and now Chimchar is Ash's first choice in battle, and in the Rotom episode was left out of its pokeball for a fair bit, whereas Buizel dissapeared after its Byron thrashing, and has only recently been seen agaisnt TR.