GTA IV may be simply too large to succesfully carry DLC, Wedbush Morgan's Michael Pachter's told VG247, as the majority of owners never actually finish the main game.
Pachter was talking in response to comments made earlier by AmTech analyst Ben Schachter (via Industrygamers and Joystiq), who expressed "concern" about the state of the GTA franchise in general thanks to The Lost and Damned, Gay Tony and other related games failing to match expectation.
"Ben's right that the DLC disappointed, but I suspect that the reason is that GTA games are so massive that a substantial majority of the GTA audience hasn't finished the core game, so they don't feel especially compelled to buy the DLC," Pachter said.
"GTA IV was at least a 50 hour game (for me, probably more like 100 hours), in contrast to the typical Call of Duty, which is a 6-10 hour game. Almost anyone who bought CoD finished it, and if they liked it (that's a high percentage), they were interested in DLC. On the other hand, my guess is that fewer than 20% of GTA owners finished the game (100% of VG247 readers, of course), so fewer were interested in DLC.
"I think that is a fair assessment, and don't think that the slow pace of DLC sales reflects poor performance for the franchise."
They're watching, kids. And they know you're hardcore. No further GTA games are public at this point.