A lot of it probably comes down to IP. A lot of their games contained graphical or sound (especially sound) assets that they licensed from outside sources; relicensing those to GPL would be impossible.
Additionally, some of the games distributed by Ambrosia were written partially or entirely by outside developers. It's not clear if Ambrosia even had the source to all of their games, let alone the rights to relicense it.
In that case I would say the same recursively to all of the different vendors and contractors involved, but that quickly becomes a much more complicated problem than someone just dumping the source online.
Yep, and it's massively complicated by the age of the software. Most of Ambrosia's catalog was released in the 90s -- most of the consulting groups involved have probably long since been acquired or dissolved, any contact information Ambrosia had for the developers is probably stale, and some of the developers may no longer even be alive to grant permission.
I don't think that any one person or group has all of the IP and they can't be persuaded to communicate enough to release it. I get the sense that it was pretty freewheeling back in the day.
At this point, what would be the downside to open-sourcing the original code and putting it under GPL 3? Why hold onto IP that you aren't making any money off of?