At some point, some years ago, it featured the best compression levels among the lossless audio encoders. But then development halted and competitors like Monkey's Audio, OptimFROG and Lossless Audio caught on.
It was closed source. Added to that, it was very slow on decoding and had few features. So, it never became a really popular format.
Recently, RKsoft was revived by Mark Taylor, now with the name MSoftware, so there is hope RKAU development will start again.