I don't think the problem is that we can't identify routes that would get a lot of use. It's that people running public transit have been charged with balancing those high-usage routes with service that's meant to serve as a social safety net, so that people aren't left completely without any transit. These are very different goals, and because transit agencies are not typically funded well enough to do both well, they are often in tension.

Jarett Walker writes well about this coverage vs. ridership tradeoff: https://humantransit.org/2018/02/basics-the-ridership-covera...