Quite often, especially with this colour of Dralon, the acrylic pile is nice and clean, but the cotton backing has absorbed some moisture and is showing itself as being darker. This gives the effect of being soiled. The more you clean it, the worse it the worse it becomes. You'll probably find that it's fine when it's dried out.
When working, to check on quality of clean, mist some M/S on the fabric and towel off and check for any further soil transfer.
Safe and happy cleaning

Ken