Long time ago I asked on their forums if they are getting anything from gog and I forgot about it :P
But I checked recently and this is their answer:
"In theory we should have received royalties on all Giants related software. In practice we never received a dime, hence no sequel."

I always thought that old Devs earned something from sales of their game. If it's not them then who takes the money?

Well given the game was published under Interplay, there may have been something in their deal that would allow them to get royalties, but is no longer valid due to Interplay's downfall in the late 90s.

Pure speculation is the best you will get.

The game sold really poorly. My guess is that the sales never recovered the budget for the game, so nobody made any money from it.