More that he doesn't have anything else that he can get him on.  
Edit:  Ok, you got me, Mandark. 
Wait, it's good because Bernie's policies are actually popular.  The most superficial things that you could make fun of Sanders (hair, waving arms) are more like a trademark so he sort of has Trump like armor there.  He's perceived as authentic and honest so that closes other lines of attack.  Calling him a socialist is a bit of a so what.