They gave an actual explanation it seems
To just about everyone’s surprise, Blizzard actually agreed to meet with key personnel from the “Nostalrius” project after shutting down their legacy World of Warcraft servers recently by way of a cease and desist.
Project manager Viper reported that five members did indeed meet with Blizzard last Friday at their Irvine campus, and discussed the issue at large over a “five hour meeting.” And it wasn’t just a token gesture either, as CEO Mike Morhaime and eight other members of the WoW team (including the executive producer and director) were in attendance.
Long story short, Viper notes that it didn’t feel like a “PR damage mitigation move,” and was instead a real discussion, leaving Nostalrius with a “deep respect” for the publisher at its conclusion, and without an NDA to sign (very interesting). The gist is that while Blizzard does have the source code for the “vanilla” (base) version of WoW, it would be tough to actually implement.
Viper explains in detail, stating: “in order to generate the server (and the client), a complex build system is being used. It is not just about generating the ‘WoW.exe’ and ‘Server.exe’ files. The build process takes data, models, maps, etc. created by Blizzard and also generates client and server specific files. The client only has the information it needs and the server only has the information that it needs. This means that before re-launching vanilla realms, all of the data needed for the build processes has to be gathered in one place with the code. Not all of this information was under a version control system. In the end, whichever of these parts were lost at any point, they will have to be recreated: this is likely to take a lot of resources through a long development process.”
So in the end, no promises were made, but with a community one million strong (and that’s just Nostalrius — I would buy into an official Legacy server with upkeep) that can potentially be converted to paying customers, Blizzard is probably eyeing this concept a little more closely. Either way, this is a really cool thing for them to do, especially since there were no NDAs involved.
Meeting report from our post-mortem presentation [Nostalrius]