Just a guess, but it might have to do with keeping the soup people happy.

Arreola is the #2 contender for Wlads IBF belt, behind Povetkin who in injured. Haye is not ranked by that body. Arreola is also rank #3 by the WBO. Haye is ranked 2 by that body, but fighting the number three probably makes them happy enough. Since holding those straps matters to Wlad, this gives him a chance to make a defense that will satisfactory both of those bodies. Haye is ranked 5 by the WBC which is probably high enough to convince the WBC to make JC Gomez wait until after a higher profile fight happens.

I wouldn't take too much from Haye's win over Barrett. Barrett isn't half the fight he once was, and he was never that much to begin with. Haye is in for world of shock when he steps in with Vitali.