In reply to Clarks0oñ.

Melody didn't join the group because she's a singer, not a dancer and she already explained it.

Nicole has always been the lead singer, they presented the band in that way because they wanted to do it that way, there's nothing more to say so stop asking why the others don't sing as much as her, that's what is the Pussycat Dolls, a singer surrounded by dancers, end of the episode.