...Besides, it did get a spin-off Puzzle game and a full Gameboy remake.
Still a pretty small franchise, all things considered. There's kind of... 4 games not counting straight ports. 5 even more technically. Quest 64/Holy Magic Century are the same game on the 64, and Eltale Monsters is a full remake of Quest 64 with tons of changes. Quest Fantasy Challenge(which also got the name Holy Magic Century for the UK port) is a puzzle game, and basically a redeco of Mr. Do!, but with more monsters/items. The last one is Quest RPG(and a slightly updated version in Jack no Daibouken, the Japanese version) otherwise.
...It gets confusing. Either way, the franchise went for a while. The only thing cancelled was Quest 64 2, and that was due to bankruptcy issues. Fun fact too; Quest 64 was considered a success sales wise, which is why a sequel was greenlit to begin with.