I think the reason why MLP was so successful is because there really is a pony for everyone. The blank slate given to pony is just ripe for self-insertion and feeling connected to a character. I mean, I saw alot people working in the medical field really like the nurse pone.
It was just so damn accessible. You didn't even need to really like the show to find a pony you could identify with. The choices were seemingly endless.
With that said, its true, this show does have limited appeal. I'm not deluding myself that it'll ever get as big as MLP, but seeing as I used to be a huge fan of that show, I secretly wish SVTFOE was nearly as large.
But then again... maybe its better this way.