Because it's an era from over a century ago; most existing modelers don't model that era; and most new modelers are interested in prototypes they were/are exposed to in real life.
The third point is actually the case with most modelers.
Actually, a lot of the early wooden passenger equipment hung on into the 1940s and even the early 1950s, so you don't
need to be modeling 100 years ago to need these cars. For my own personal situation, I am still Jonesing for
some single-window heavyweight coaches, and some wood "palace" type cars, because the SP&S was still running
some of those into the late 40s and they would look good behind my recent passenger steam projects.
I do, however, completely agree with your premise.
It was "a long time ago" and many new modelers aren't
going to be as interested in what they never saw in real life.