It is because women weren't really able to compose back when our favorites were composing. Mozart was definitely better than his sister, regardless of her gender, so it seems only fair that we listen to his music. There probably were geniuses in Mozart's level who were female, but none had the opportunity he did. Today, most conductors seem to be male for several reasons. a) a lot of the ones we know are older, and therefore, in their time, had the gender biases at their advantage. and b) women generally, especially after 2nd wave feminism seem to be inclined to other professions, such as medicine.

A composer, and a good performer, be it singing, or playing, requires, usually, an early start. Unless someone is brought up to become a virtuoso, they will not be one. The same with conductors. The same with composers. That being said, there are some excellent female musicians, and female singers are just, if not more, popular than male ones. The problem really is that orchestras play canonical works, which were written in pre-feminist times, and therefore, are less inclusive, than modern music.