I don't think others were not willing to hire female designers; some Infocom games from that time also had female authors. I think it's more to do with the time they started. Somewhere during the 80s the 'white male nerd' stereotype became a self-reinforcing thing which scared a lot of women away from everything computer-related.
Also, I don't consider Sierra games 'the best designed'. Some of their games have lots of 'personality' and they were first with many innovations, but it's clear everyone was still figuring out how to design games at the time.