The main Sierra "Quest" series were re-released twice post 2000, with Vivendi re-releasing the 2006 Collections of KQ, SQ, PQ and LSL; The same games were re-released for modern PCs by Gog.com last year. Gog also re-released many other Sierra games like the Gabriel Knight series, Phantasmagoria series, SWAT series and quite a few others
But why hasn't Quest for Glory been re-released all this time? It was arguably in the A list of Sierra series along with KQ, SQ, PQ and LSL in terms of popularity, and quality wise was just as great as those. It's one of Sierra's series that really deserves to be re-released so it can be played easily on modern PCs.
The same goes for the Conquests series, Torin's Passage, the Shivers series, and many of Sierra's mid 90s adventure games like Lighthouse, Rama, and several others. All deserve a re-release on GOG.