Yeah, it's an unfortunate niche. I think it's the sort of thing where there are companies that would be happy to contribute back code, but only if a CAD program were already at 80% of their needs. But without commercial support, the hobbyist-CAD community is way too small to get a system to that 80% to begin with. Lots of industry doesn't really enjoy paying large per-seat license fees for the dominant CAD systems, so I think they'd be willing to work on an open-source one and contribute back, the way embedded-systems companies do with Linux, but they aren't willing to take a leap onto something that isn't there yet

BRL-CAD's particular problems are because it's sort of a time-capsule project, I think. It's an Army project from the 1980s that was open-sourced in 2004, but is still very "1980s Unix workstation" in a lot of ways, and the ratio of interested developers to giant crufty codebase has meant that modernizing it is slow.