> But in most cases it was likely because it was extremely difficult to be published as a women at the time.

I don't think that is true, since the publishers knew the identity of the authors (as far as I know). And there are some examples of women authors being very successful (e.g. Harriet Beecher Stowe).

It is more likely they used a male pseudonym because they didn't want prejudice against women and women literature to affect reviewers and readers, and/or they wanted to protect their privacy (and reputation, if they were writing about scandalous subjects).