Nursing a baby and being able to unbutton with the right hand while holding the child.
Of course, not all women are right-handed but majority often rules.
That might be a good reason, but I can’t find any historical evidence for this being the reason. Further, women’s shirts would require the left hand to be used to unbutton — men’s shirts are set up for right-handed buttoning.
The most commonly accepted explanation is that, in the time period when buttons first came into use, they were for clothing of the rich, and rich women did not dress themselves. So, the buttons on their clothing were set up to make it easy for the maid.
Which also makes it easy for the male, but that doesn’t seem to be the purpose either.
Although I am left-handed, and have no trouble using my right hand to do buttons. It doesn’t seem like a complicated task to learn.