Perhaps someone could care to educate me on this one, or if there were any "politics" behind it.
It's mostly US and EU games that don't have VGA support enabled.
My guess: Didn't care / didn't know / forgot to.
There are exceptions though: Some of Capcom's Japanese Matching Service games don't support RGB nor VGA (RGBHV), which some assume was meant to prevent people from using them in public locations like arcades, cafes, bars, etc. pp.
European games that were optimized for 50Hz display (like Code Veronica) don't support VGA because it would mean that it wouldn't comply with the VGA standard anyway (640x480p60). But even those could be patched/tricked so they'd at least work on some European CRT monitors - but they wouldn't work on HDTVs because they wouldn't accept 50Hz via VGA.
Surely this would a catalyst for running a "backup" version of a game (instead of a GD original) as you could at least manually patch those to force VGA support?
All that's missing would be a flag in the IP.BIN bootsector, so yeah, they could be "patched" by adding this flag.
So what dos that mean some games won't work with my vga adapter or they will just run on SD rather than HD?
Games that don't boot in VGA can be tricked to work in HD by simply booting them with the VGA adapter/box/cable unplugged. Once the cable is plugged in, the Dreamcast switches to VGA mode.
This list is in an odd format, I could share it it would just take me a while to convert it.
If you could post it in DCEmulation's Wiki it would be appreciated