[quote=Azarthes]
It's cause they aren't.
[/quote]

They're a thing.
They're just far less popular for two big reasons.

1. A lot of people are raised to see women as the victims, not men. So where feminism get's a lot more support, MRA get's a lot more blind eyes and dirty looks.
2. Feminism has boosted in popularity with shenanigans such as Five Guys, Feministfrequency, Oh look! Someone said something mean on the Internet! White Straight Cis Male Privilege etc. Getting a lot of attention both from SJW looking to join the bandwagon, and people who are outraged by the bigoted behaviour and false information being spread by feminism. While MRA doesn't tend to start that sort of stuff, therefore not as much attention or popularity.