Possibly a historical artifact; fish was created in a time when the controversies surrounding lilo were fresh in memory and lilo was still boss. Actually there is a channel on freenode now, but it's about a third in size.

Other examples of projects officially on OFTC include: debian, tor, open street map, libvirt/qemu and related tech, ceph, awesomewm, gcc, smuxi... So fish isn't all alone there.

There are also projects that operate their own networks, such as GNOME and Mozilla.