That's basically the reason why I'm not into brony music at all. Maybe it's cause this musicians are infulenced by songs from the show itself, whick are... meh... I mean, they're not bad. They're just simple and alike.
But more likely the reason of this could be the lack of platform that would allow brony-musicians to get money and some worthy attention for writing their songs. Not for the sake of money itself, but beacause writing and producing instrumental music takes lot of recources. Electronic music is cheaper and easier to make, you don't even need an actual instrument or good sound recording devices.
And, sadly, Hasbro would not allow this kind of platform to exist.