Description:

Basically, because there's not a complete one. kmix is the official mixer app for KDE, but it lacks:

* Support for enumerated items, when the control is not just a volume, but a choice between different elements
* Support for splitted playback/record volumes: sometimes, an element can have different volumes for playing and recording: kmix doesn't show them all
* Support for balance: one cannot adjust Left/Right volumes in an independent way
* You cannot configure what you want to see or not; for 90% of the people, "IEC958" item is useless and correctly hidden, but for someone it may not be

With this, I'm not stating that kmix is not good: it is, indeed, and it works in 95% cases; but, you know, it's a matter of choice :)