I think there's no reason beyond the evolution of the design itself.
This is what I think about usability in terms of design:
Asymmetry: It's well known that asymmetry is perceptually more unsettling, thus offering greater visual impact.
Spaciousness: The hamburger menu is used more frequently on small screens where saving space is a fundamental premise. Although the width of the icon and the thickness of each line may be the same, the difference in the length of the lines gives the feeling of spaciousness to the area where it is located.
A simple test in Google image search:
Hamburger Menu Icon
Modern Hamburger Menu Icon