troff has a substantial history. Significant
changes in troff could invalidate most of the old documents leaving troff
with no usage base, and a poor tool at rendering all of the troff documents
out there.
As a living example, I have troff files from as far back as 1975 that
still work, and perhaps some even older that have lost their dates due
to careless copying. The only incompatibility with groff is easy to
fix: inserting a space before the arguments of a troff request. The
few other incompatibilities I've encouuntered have been graciously
corrected by groff maintainers. You get no such help for old Word files.
doug