Anecdotally, a project I worked on involved evaluating some encrypted file systems on linux and encFS (on FUSE) was at an order of magnitude worse performance than kernel solutions.
Not to say it could never happen, but the tradeoff on performance is just no where near worth it right now.
I'd suspected performance might not be the primary purpose when I came across the Python packages for implementing filesystems.
Still, it seems like a lot of the interesting filesystem ideas have to do with non-root users authenticating to something else over some network connection. This type of thing is simply a more natural fit for user space.
Keeping the high-level stateful protocol stuff out of the kernel is usually a good idea except when performance is all that matters (i.e. there will be full-time developers tuning it and cleaning up the inevitable security and crash bugs).
Not to say it could never happen, but the tradeoff on performance is just no where near worth it right now.