FUSE is an informally specified one-stop library that explicitly targets the Linux VFS, so it might be considered more convenient for whatever reason. There's also a general lack of knowledge about Plan 9 and Inferno on anything beyond a superficial level, though this might be improving.
9P is a small generic protocol that is often independently implemented because of its simplicity, so there are many heterogenous libraries for it.
Which required significant kernel additions in the case of the BSDs, and is a third-party kext for OS X.
In contrast, 9P is just a byte stream protocol that ranges from weekend project implementations in ~1000 LoC (yet still perfectly usable) to full strength client-server libraries. It's much more versatile and can be done entirely in userspace without special kernel cooperation beyond a basic POSIX runtime.
9P is a small generic protocol that is often independently implemented because of its simplicity, so there are many heterogenous libraries for it.