Most of the 'natural' fibers just feel better to knit with.
There are many acrylics out there that are soft, but after working with the soft natural fibers, the acrylics feel like plastic to me. It's a shame, because acrylics are good for a lot of things--I just can't stand the feel of it anymore. I can use a blend, like Encore, but straight acrylic, no.
Superwash wools are machine washable, and they're very soft, so you get the best of both worlds there.