I feel like a lot of people that aren't masculine men grew up seeing, both in their lives and as depicted in popular media, masculine men as bullies. As such a huge portion of society - women and men who aren't masculine - grows up with a negative opinion of masculinity.
This bias against masculinity has spread as women and femininity have become more prominent and powerful in society.
The feminine attitude is opposite the masculine in many respects, so it's natural that the two would dislike one another and come into conflict.
As our society shifts to one in which the feminine is embraced rather than rejected, and the masculine continues to be seen as a menace to society, it's considered more acceptable to mock "bros" than a female or feminine equivalent.
Edit: both the feminine and masculine aspects of personality have their strengths and weaknesses. Neither is really better than the other in some objective sense. Different situations call for different attitudes.