My theory is that it has characters almost anybody can relate to. There arent many girls who get down in the dirt like Applejack & Rainbow Dash. There arent many girls who do nothing but read like Twilight Sparkle.
Pinkie Pie & Fluttershy? those 2 girls have another side of them; both are SCARY. D:
Im female, but i consider myself a "brony" because damnit i can do that.
i like the show, because i can relate to each character in some sort of way. Fluttershy loves animals, RD is tom-boyish. Twilight loves reading, Rarity is quite the artist who doesnt like to get dirty. AJ is truthful, Pinkie is full of fun. i think this aspect of the show, the fact you can ACTUALLY relate to a specific character thats FAR different from the next, is what draws in men.