So if you'll notice, both times Kitty fought Diane, she was being defensive / evasive. The most obvious reason is because she knew she couldn't beat the Crimson Paw, but some people have also speculated that it's tied to Kitty's backstory (which is technically of dubious canon since those scenes were not included), and that she didn't want to hurt Diane because she still cares about her. Either interpretation actually works pretty good.

In context to the deleted scenes, it also makes sense that Kitty was able to beat Wolf but not Diane, since Diane trained Kitty for presumably a few years but has had a lot less time with Wolf, and neither of them can beat Diane.