In my brief study of quantum mechanics I have found that math done using operators typically breaks from traditional mathematics. Almost certainly between linear algebra and probably something else there is plenty of mathematics to accurately sum up operators. That being said, does it make sense to ask questions about the math of operators on math stackexchange when the math is describing entirely physical systems?
For example if you have the operators A and B that both correspond to observables and you have a question on a bit of math involving them, doesn't it make more sense here where you can simply say that you are doing quantum and one immediately understands how to work with the operators? If you asked in math you would have to 1) be able to appropriately tag your question under the relevant fields of math (who knows what they are) and 2) have to explain very well the background to where the math was coming from.
Furthermore, sometimes things such as the canonical commutation relation throw a wrench into the works in a way that mathematicians might not pick up. For example when helping someone from a physics standpoint you would know that problems involving x and p might lead to having an i hbar d/dt pop out somewhere. Whereas in the math forums you would have to explicately remind people that this is something that exists that they could potentially work with.
It just seems like most math questions that stem from quantum mechanics are much more relevant on the physics forum than on the math forum.