Downloadable content was a concept so full of promise. Those of us who remember the early days of DLC can recall the high-minded ideals of the proposition. Play more of the games you like and stave off unnecessary sequels in the process! It sounded like a win-win for gamers and publishers alike. Years later, as the concept has slowly become standard practice, DLC announcements are generally met with a shrug. Why don't we care anymore?
We can partly fault the publishers. In practice, DLC hasn't usually prevented sequelitis or enhanced the game experience. It's used more as a sales tool to push pre-orders, undermine used game trade-ins, and serve as fodder for the inevitable "Game of the Year Edition." Each of these can have benefits for gamers, but they're clearly more slanted towards the publisher side.
Publishers have not exactly been cautious about their approaches. The early days were a wild west as game makers threw everything at the wall to see what stuck, creating some unexpected issues and backlash. The most famous of these was on-disc DLC, as savvy gamers noticed their downloads were a tiny file only used to unlock content that already existed. Any explanation of software licensing law or game budgets was moot--as far as gamers were concerned, if you bought the disc, you should be able to access everything on it.
Other problems have been more subtle and pervasive. Some publishers pump out far too much DLC, or overprice it. Activision has been criticized for its regular practice of putting out four maps (three new, one revised) in each Call of Duty pack for $15. Worse yet is when the content unlocks some extra weapon or ability that makes the game a cakewalk, as Square Enix tends to do with small DLC like Tomb Raider's Adventure Pack. This is still especially problematic when it comes to pre-order DLC, which is all too happy to ease you through the first half of a game with an overpowered weapon. Meanwhile, even DLC that sounds great upon announcement sometimes fizzles in practice. BioShock Infinite had ambitious plans and a season pass, but we ended up with some combat challenges and an uneven story presentation.
It certainly doesn't help that DLC availability can be incredibly confusing. A much-publicized graph showed just how complex a DLC release model can be when it comes to various retail editions and pre-order bonus content. Even your choice of platform can create baffling situations. Think back to how often you heard Microsoft or Sony reps call some piece of content "exclusive" to their platform at E3 this year, only for word to come later that it came with an asterisk.
All in all, what could have been a way to strengthen communications between gamers and publishers by giving direct feedback on content we want has more often been used to manipulate or goose sales in the publisher's interest.
The advent of DLC has also brought about a new game-changer: free-to-play. By offering content for sale, it was only a matter of time before the content became the main revenue stream, forcing the game itself lower and lower. The traditional $60 retail model hasn't gone the way of the dodo, but microtransactions are a subset of DLC that have arguably made a much bigger impact.
Our collective disinterest could be much simpler, though: it may be that we just don't care all that much about more of a good thing. As anyone who has checked out the deleted scenes or "Extended Cut" of a movie can attest, sometimes an experience feels more complete as a smaller, tighter package. Honestly, after playing through a 30-40 hour game like Watch Dogs, do you really have that much need for another few missions?
Obviously, some companies are still doing DLC right, and getting rewarded for it. Downloadable content can absolutely be worthwhile. The idea was bold enough to shake up the industry, and it still could be a powerful tool if done right. If DLC can get back on-track at all, it will only be possible if publishers restrain the easy buck and use it to fulfill its original promise of iteration and communication with players. As long as they're merely treating it as a sales tool, that's how gamers will see it too.