Girl bands aren't as successful as boy bands. Besides a few, obviously like Destiny's Child // Girls Aloud.
Most fade away after an album or two.
This is mainly because of the general public they aim for. Girls love boy bands because their hot/cute and omg let's go crazy i.e. "MARRY ME//HAVE MY BABIES."
Girls don't really support girl bands the same way (cause they can't be obsessed with them, unless their lesbians) and male fans are usually gay or just open with their sexuality. I don't know many straight guys who obsess over girl bands
A girl band = gay male fans mainly
A boy band = tons of females & gay fans mainly