I think it has mostly to do with cultural norms. Women are typically better at English, because we’re raised to be communicators, to understand how people are feeling, and to express our emotions. Writing is a part of the arts – a way for people to express their feelings. Stories, poems, love letters, etc. Men, on the other hand, are not raised to express themselves the same way, typically.
Also, go far back enough in time and try to decipher a male’s handwriting from a female’s. It would have been nearly, if not completely, impossible. Penmanship, however, used to be important for every literate person. Letters themselves used to be much more artistic, and as the Western world progressed, artsy and “flowery” stuff became a female stereotype – so it makes sense that a lot of modern men don’t have nearly as nice handwriting.