Sexism

[addy]

why is it that all or most maids are women?

It's likely due to culturally perceived gender roles, where women not only make good housewives, they're also weaker and docile so safer to let into your home than a male stranger.

I suppose an argument could also be made about biological differentiation between the sexes, as TS pointed out. The stereotype of females is that they are more detail-oriented than men, so presumably they'll be more finicky and orderly about the house.