I'm guessing it's because you can't write a deep and emotional classic song about multicolor cartoon horses, and genres like metal and hard rock would be too brutal.

That 99% of brony musicians stick to electronic, probably.

Which is untrue

Because it's a figure I pulled out of my ass for effect.

IF you want to be nit-picky, I'll say that most brony musicians tend to stick to electronic in some form or another.