Kmixer is not at fault, and kmixer is actually capable of supporting just about any sample rate your sound card might support.
What happens is the sound card/chip has a PLL circuit which acts as the internal clock to lock in the sample rate. The AC97 codec requires support for 48 khz. So most of these AC97 sound card/chips would just have one PLL circuit which only runs at 48khz or perhaps multiples thereof (I assume this was chosen due to movie audio being at that sample rate). So it is a complete hardware limitation, and everything must be resampled to match this timing. Of course, they could have included a separate PLL just for 44.1khz, but it would be additional expenses which weren't considered worth it at that point in time.