I theorize that the reason /co/ likes this so much is because of essential sexual balance. Every human being contains aspects of both male and female, despite their gender, and need some catering to both. The natural /co/ denizen is swathed in masculine elements, in porn and beards and robots and fart jokes, as well mostly keeping to a circle of males as far as friendship goes.
This show provides a shot of estrogen that fills a hunger that most people were not aware they had. In short, even the manlies men need to be girly sometimes, and /co/ is not made of the manliest of men. MLP's good animation, voice acting and presentation, in addition to its capacity to be enjoyed in privacy, allow indulgence with less shame.