By 1993 SSI was branching out in other directions.
After 13 games in five years in the same basic engine, this is understandable. In fact, aside from interactive fiction games, this could be a record. I can't name any other series of games that released 13 titles in five years.
I am not too sure how much longer they would have stayed with the 3D exploration/overhead combat format of the Gold Box games. They may have finished their pre-existing commitments in the Forgotten Realms, Krynn and Savage Frontier series as soon as they could, so they could finish with the Gold Box format. Savage Frontier stopped at two games!
This is incorrect. There were three games in the series, most people just don't realize it. The second one is the original "Neverwinter Nights," which is why the first game takes place in the region of Neverwinter, so that it can act as an introduction to the area. They were hoping that if people knew that area already, they would want to play "Neverwinter Nights."
So, they considered the order of the games to go this way: Gateway to the Savage Frontier, Neverwinter Nights, Treasures of the Savage Frontier. Little known fact: Treasures was a sequel to the MMO!
Additionally, they saw the "Dark Sun" games as the evolution of the Gold-Box engine. It was the engine that they tried to replace the Gold-Box engine with, which is why those games are a lot like the Gold-Box games. The problem was that people didn't like it as much and everything was moving quickly to 3D...otherwise, we might have seen 13 games in that engine as well!
Source here: http://en.wikipedia.org/wiki/Savage_Frontier_(series)
And before anyone debates that there were 13 games using the engine, here is the List:
1) Pool of Radiance
2) Curse of the Azure Bonds
3) Secret of the Silver Blades
4) Pools of Darkness
5) Champions of Krynn
6) Death Knights of Krynn
7) Dark Queen of Krynn
8 Gateway to the Savage Frontier
9) Neverwinter Nights
10) Treasures of the Savage Frontier
11) Unlimited Adventures (Heirs to Skull Crag)
12) Buck Rogers - Countdown to Doomsday
13) Buck Rogers - Matrix Cubed