A true soul voice solo seems to be a burden. Being in a group seems to relieve a lot of expectations, to where f(x)'s Luna was doing the Coyote Ugly and Legally Blonde musicals, neither of which have music styles that her voice would seem to be suited to. They also have the liberty of playing their voices off of foils in their fellow members' voice, like how Jea's fire in Brown Eyed Girls' "Sixth Sense" is so effective because you have Miryo, Gain, and Narsha to contrast.
Would you consider 2NE1's Bom to have a true soul voice, or is it too congested?

Also, Asia has this shameless thing for sappy ballads. So in that sense, there will always be more of a place for a true soul voice in Asia than in the west, even if a shallow one.