Mixtes were wildly popular during the 1973 to 1984 era, here in the United States, as sporty drop bar 10 speeds and 12 speeds for women.
Perhaps nearly 90% of those were with racing style handlebars (drop bars) instead of the tourist style used for upright city-tourist riding.

Mixte frames are great, but they fell out of favor since they weren't diamond shape which is more easily carried on a typical automobile bike rack.
Fewer manufacturers produce them today. Stupid idiots preached and continue to preach that no pro rider or serious rider would ever consider a mixte frame.
This idiot mentality keeps the mixte frame from being considered by more than only the old & middle-aged men who seek them out for ease of mounting.
All bike riders in general have been conditioned or brainwashed that mixtes can never be as good as a diamond frame.
In addition to the automobile rack mounting issue, women don't consider mixte frames today largely because they are led to believe this 2nd rate myth garbage. They do not want to take a chance on buying a bicycle that isn't considered acceptable among their LBS ride group and their peer group of riders
Old farts (older men who aren't as spry or flexible as they were when they were younger) typically migrate to larger classic mixte frames when they reach the point of having difficulty mounting a traditional diamond frame. Most men feel like it is somehow shameful to ride a " girls bike" but since they enjoy bike riding they don't wanna stop doing it until they actually cannot do it in some way. Somehow these bozo idiots think somehow it is less disgraceful to be seen on a mixte than to be seen riding on any step through frame Bikeforums members are less like this than The Cabe's forum members, many of whom would never ride a girls bike, and take the unwavering view that a girls bike is only good for a parts bike, or for a girl to ride (wife, girlfriend, daughter, grand daughter, sister, aunt, mother, grandmother.....etc)

Yes, the fact that mixtes and step throughs are more difficult to mount on an automobile bike rack is a very valid reason not to choose one, but that is perhaps the only negative issue if you are simply looking for a quality, easy to mount and easy to ride, comfortable bicycle.
The beneficial side of this "lack of love for mixtes" is that nice old used mixtes are typically priced much lower than similar quality classic diamond frame bikes from the same era, and/or from the same manufacturers. This means that likely you won't need to pay much for one, if you can find one in your size.
Old men have been taking these and removing the original racing style drop bars and replacing them with tourist style handlebars in a style nearest to something like the old Schwinn 7881 handlebars. Never underestimate an old man with a mixte or a step through. It is no longer Sally riding it as she did when she was a student at University of Virginia from 1975 to 1979 when she graduated. Today it is an old man named Sammy that at 68 years old today, he needs the mixte because he cannot easily mount a diamond frame now. Sammy didn't want to, but realized there was no other way around it and he still loves to ride, even if it means riding a girls bike like a mixte. Sammy understands that there is less of a stigma to be seen riding on a mixte than a step through for whatever reason. .....Ride Sammy Ride!