Main reason is the loss of the duality between man and mask. Look at the recent relationship of Pete and Carlie Cooper: Like it or hate it, the fundemental stressing point and what ultimately dissolved it was that Peter hid his spider-man persona. Super-heroines don't have that conflict. (Only exception being Black Cat because she's meant as an inversion, where she actively doesn't want to know about the man under the mask).
That said, if they ever did, I'd love to see Carol/Peter.