Sierra was planning to produce Space Quest VII: Return To Roman Numerals, but produced something called Grim Fandango in the meantime (some say Grim Fandango was a good game, but the only advertising for Grim Fandango I recall showed some blocky looking character with a crude face drawn on the block’s head and very little description of what the game was actually about).
Sales of their crappy Grim Fandango game were “disappointing,” so Sierra decided to spite themselves and also punish their Space Quest fans at the same time by cancelling Space Quest VII.
There have been attempts by fans to write their own indy versions of Space Quest sequels, and some are available for download.