Greg A. Woods writes:
Ditroff (or sqtroff) was also incredibly rare to non-existent for 99% of
the Unix sites I worked at and visited; even some time after it became
available. Even sites running native AT&T Unix, e.g. on 3B2s, and thus
could easily obtain it, often didn't want the added expense of
installing it.
Maybe for you; I had it everywhere that I worked.