My theory is that being a leftie stopped being an issue as fountain pens slowly got replaced by pencils or ball-point pens. As a leftie, I have never been able to successfully use a fountain pen. This is because the ink has not dried fast enough as my hand moves left-to-right over the paper. Smears all over. When writing with quills dipped in liquid ink, I imagine it was even worse.
The roller ball pens are the closest to a fountain pen experience I will ever get, thanks to modern ink technology.