Going over old reports, apparently when Telltale first announced that they just obtained the licensing to make Sierra games from Activision (which they announced at a press event in San Franscisco on Feb 17, 2011), their license included several of the games of Sierra's back catalogue. King's Quest is apparently is just the first game series they planned to work on.
Telltale has entered into an agreement with Activision, current owner of the rights to the classic Sierra On-Line adventure franchises, to create new episodic games based on these series. The first will be King’s Quest.
Seeing as it was only an initial anouncement made at the press event, they never gave a major press release on their website. They only made a single reference iin a blog post (on Feb 18, 2011) to how King's Quest was going to be one of the next IPs they were going to work on after they finished Walking Dead and Fables. No press releases nor websites were made for King's Quest or the Sierra license in general.
Since many of the games they announced got pushed back from their initial release dates, and this has affected their entire release schedule. This has probably affected any development on King's Quest as well.
In anycase I think its interesting that they apparently have the license to make other Sierra game franchise games. Perhaps this would if they continued to include, new games within the Police Quest, Space Quest, Gabriel Knight, Quest For Glory games, etc. If Adventure Gamer's account of that press event is accurate.
Obviously it would have never included Larry, as that was already sold to Codemasters. But the series I listed above are the major series that Activision has rereleased through GoG so far.
But it seems that the announcment at the event was apparently more of a general reference to them obtaining the Sierra family license.
Think about it if they have exclusivity, that could affect all the Ips I mentioned above! Maybe a few more!