I don't know about Diablo 2, but Blizzard is so shady and messed up nowadays that I wouldn't be surprised that they "lost the source code" to prevent modders being able to port games, etc.
As for transformers, it was never lost (PCGamer, if you don't like Xfire). Hasbro claimed they wanted to provide access to legacy games, but completely made up that the source was lost. Now that we know that the source is still available... well, Hasbro clearly hasn't tried to rerelease those games.
(note: I know this is the same company, Activision Blizzard in both cases. For anyone reading who doesn't know, they were not the same company for the release of Diablo II, and a good amount of time afterwards.)