Some other already gave good possible explanations to this, but I am adding my own subjective uninformed view on this:
Not many people may actually like wearing crop tops but they do it for 'fashion' reasons and those fashion reasons so far dictated that women are the ones who have to wear them.
Me, as a man, haven't personally tried crop tops, but it feels to me like it would be uncomfortable. It feels actually uncomfortable to me when sometimes I wear an old t-shirt at home which has become shorter leaving some small lower parts of my back or abdomen uncovered. And it's not because of any social construct, I live alone and nobody can really care about what I wear, so it's not that. But it's like feeling cold on the lower back of my torso but warm in the upper part. It just feels uncomfortable.
That's just my personal feeling but I can imagine more people could feel the same.
So I can imagine wearing a crop top can give a similar uncomfortable feeling?
But sexualization of women required them to expose more parts of their body (most of their torso) while covering those ones not considered to be decent enough to be shown in public (breasts). But that sexualization and exposure of their bodies is something that is usually not so much required from men.
I think the original question asks why not so many mean wear crop tops as a choice they make, but I think it hasn't been so much a choice for women as it may have been a command from sexualizing fashion and the heteropatriarchy has determined that the uncomfortably and exposure of their bodies related to crop tops is something women have to wear not always because it's their choice but to comply with sexualized fashion standards.
I am not a woman or wear crop tops either, so I may be wrong on all this, I'm mostly just thinking out loud 😄