In today's market, new IP's are a huge gamble. Most don't pay off. Consider that, and then consider that of all the new IP's of the last five years or so, Borderlands is easily one of the biggest breakout success stories.
And yet, not much is really happening with the IP. IDW has done a few comics. That's about it.
For those not familiar, it's a class-based co-operative shooter set on a mined-out, post-boomtown planet. It has an atmosphere that evokes spagheti westerns and Mad Max movies. Its gameplay is driven by XP and loot in the tradition of Diablo or Torchlight, but with a focus on guns rather than archaic weapons. Playing the game, it becomes evident how much more diversity can be bound in guns than in muscle-powered weapons. Sure, a sword is faster than an axe, and axe hits harder, but there's not that much there to mine from a design point of view.
Now, think about the distinction between sniper rifles, shotguns, assault rifles, and rocket launchers. You have damage, recoil, rate of fire, magazine capacity, reload speed, scopes, and more. And then there's different types of ammo to boot.
Borderlands is just screaming to have a Descent-like treatment.