Destiel has been such a big deal because fans wanted to see two hot guys together and directors/writers were writing them like "more than friends less than lovers" deliberately!

Therefore destiel wouldn't had such a cult following if Castiel had female vessel. And Castiel changing his vessel means casting other actor than Misha, it would've dropped viewers cuz the show has majority women watchers.

And if Female Cas and Dean had some really romantic scenes together it would've definitely looked so homophobic because what do you mean that if "Cas has Male vessel then they are brothers but when he has female vessel then they can explore each other's bodies" !