Because to some people "mens rights" sounds like "white's rights", and the idea that men are now suddenly experiencing massive discrimination is about as believable as the idea that heterosexuals are now being systemiatically persecutre for the sexual orientation.

Also, for every 1 law (or societal norm) that are highly skewed against men, there are 10 that are highly skewed against women. (for "child custody law", I give you "access to contraceptives", "evidence during rape trials", "chance of suffering domestic violence", "chance of getting elected as a politician", "chance of getting appointed to a board", "chance of founding a start up", "frequency of being viewed as not a hacker/programmer", "risk of rape", "chance of being the victim of sexual aggression in the workplace", "chance of being the victim of unwanted verbal sexual attention on the street")

On average, men are still in a much more privileged position than women.