Probably the easy ones he can prove/attempt to prove. Like he either never even met them or can prove he wasn't in the same state as they were when the alleged attacks took place. Or maybe he's just trying to scare some of his accusers away and it might cause other accusers to back off.
It could also have to do with how much damage each accuser caused. One accuser suing him in court or whatever might not cause much damage, another accuser talking to every news outlet and appearing on as many talk shows as possible would cause more damage.