Is there a reason why bikes are not unisex to this day? The only reason is some sexism well over 100 years back, where they designed special bikes for women so they could ride while wearing a skirt. No idea why men couldn't use the same bike model as women back then... some 19th century macho thing. Other stupid cases like that might exist where you least expect them. And so unisex becomes a good value label that you can stick on any gear to mark it non-stupid. Same with the "eco" trend.

@Lundin: "why men couldn't use the same bike model as women back then". No, it's not "some 19th century macho thing". Women's bikes of that era don't have the bar across the top, forcing a less desirable structural tradeoff. There is much more stress on the frame down by the crank, making the frame overall heavier for the same strength. Unless you really need to wear a skirt or kilt or something while riding a bike, you're better off with a frame layout like the old "men's" model.

The obvious answer to this question is "Because they think is sells better". Unless you expect someone from a marketing department with hard data to answer (very unlikely), this question should be closed as opinion based.

@Olin Lathrop‭ Not sure if I buy that argument, the strain that would make a difference in this case would be forces to the left/right vs the direction of travel, on the saddle bar specifically. It seems very unlikely that what little force that gets transferred to that bar would have any notable impact of the steel durability over time. The gears, chain, brakes, wheels etc will break through wear & tear many times by the time you manage to wear out the actual steel frame no matter model.

@Lundin - I don't think it's the strength that is the problem with a woman's bike - but the rigidity. The less rigid structure means more energy is expended flexing the frame rather than driving the bike forward. Note that all professional women bikers use a "men's" frame.