Probably because I haven't gotten around to putting it in yet. If you need it, [file an issue](https://github.com/gromgit/homebrew-fuse/issues/new/choose) and I'll see what can be done.
Maybe I just haven't gotten around to updating it yet, but a significant subset simply can't be updated until MacFUSE supports version 3 of the FUSE API. Read any _Caveats_ that appear during installation for important details.