Gentlewomen and Gentlemen

The phrase "gentlemen and gentlewomen" has some usage between 1586 - 1591 after which it just petered out.

At the same time "gentlemen and ladies" seems to have more prominence and that continues to this day.

The etymology of lady says "wife of a lord", so how come it's become a fit into the phrase "Ladies and Gentlemen" as opposed to "Gentlewomen and Gentlemen"?

The word "Gentleman" from that era refers to a man who does not himself have to work for a living. So "ladies and gentlemen" is a way of flattering your audience by implying that they are of a high social class (most likely higher than most of them actually are).

"Gentlewoman" doesn't quite have the same cachet, as it was far more common for a woman to not have to work than a man ("work" in this case meaning seeking paid employment from sources outside the family home). It takes far less wealth to pull this trick off than it does to have the ability for noone in the household to work. The best way to imply that a woman is of this particular social strata is not to say that she doesn't work, but rather than she is married to a man who doesn't work. Hence, "Lady".

One of the meanings of lady is woman, and it is used in polite (or old-fashioned form of) references.

I spoke to the lady at the travel agency.

One of the definitions reported by the OED is "used as a form of address to a woman (now colloquial). ME."