Because, despite what popular opinion would have you believe, we actually know a fair amount about her personality and history at this point; her relationship with her family, some of her past with Montoya, how she behaves in public as Kate and in costume as Batwoman, and that's all been in effect for three years now. There's enough there to contradict. And so far, it's being contradicted. So what now? Can we no longer trust what we saw in 52 to be accurate? Is the Holiday Special canon or not? This is a very shaky character, without much of a following or solid place in canon... so what makes more sense, starting her over completely again after just three years, or building on what's already there? Chalk-white out-and-proud rockabilly!Kate is actually a slightly *less* viable character than the cipher of 52, because it's not just a new start this time, it's a new start with a bunch of confusingly contradictory stuff shackled around its neck.