After Grand Theft Auto 4 stopped being available on Steam a few days ago, most fans assumed that expired music licenses were to blame. Today Rockstar gave a statement regarding the issue though, and revealed that the culprit is something else entirely — and also something much harder to correct.
Games for Windows Live was a much-derided, old attempt at injecting Xbox mechanics and systems into PC gaming by Microsoft. It was also poorly optimized DRM and rather intrusive with the games that were chained to it, so gamers hardly missed it when the platform got the inevitable bullet in the head.

While nobody is clamoring for the return of GFWL, its demise has caused some unexpected issues as well. Since the activation of GTA 4's game key is tied to GFWL, with the service being offline it is impossible for new game keys to be generated or redeemed, making the continued sale of GTA 4 impossible.
The game still being available on consoles was a key indicator to all of this, since music license issues would have rendered it unavailable across all platforms, however GFWL was only present on PC. Many players suspected that it was Rockstar themselves who pulled the game in preparation to migrate it to the Rockstar Games Launcher, however that too seemed unlikely since other games were unaffected.