Because the troff format was designed with the limitations on early 70's computers in mind and seem to be used exclusively for man pages. Why not upgrade to something that better correspond to how we actually code now (ie markdown)?
Remember that the worst bugs in the last couple of years have been in the code that nobody was looking at.
I haven't compared this with upstream though.
"The great strength of troff is the flexibility of the basic language and its programmability -- it can be made to do almost any formatting task." -- The UNIX Programming Environment, Brian W. Kernighan and Rob Pike.