I highly doubt you can pin it on one single thing. Take for example piracy, that does definitely have an impact on the profits of a game. However, putting it in context with (say) a poor critical reception, this effect is altered somewhat. By how much? If you know, you're going to be very rich. This effect is compounded whenever the industry evolves, as it must: Piracy expanded with the rise of broadband Internet, so DRM made an appearance. People find ways around the DRM, so they bypass it. So the DRM gets more restrictive, and so regular gamers complain about it. They post bitchy reviews, sales drop. So prices may drop, sales might pick up, and blah blah blah.
It's a continuous evolution of the industry, which must adapt to face the problems of the time. Games like TF2 have successfully experimented with a new business model, a free game (Though not initially) that now relies on micro-transactions for its main business. This policy rose (As far as I see) because sales of the game were starting to shrivel, or at least the bulk of the people who would buy it have done so.
Its also very easy to say that a developer has their priorities wrong. You could say that their latest instalment of "Generic Space Marines 9: Return of the Armageddon revolution" was a pile of shit, and you may be right. But people buy this shit. They are running a business, and they have evidently found that lots of quick, cheap, wham-bam-thank-you-ma'am games pull in a better return within a time frame than a well-conceived game that takes ages to produce. Think EA Vs Valve, You may not have such a devoted fanbase, but they still buy your products. Which of those appears to be generally better off, financially?
The ultimate source of money within the industry is the consumer. These AAA companies must have found what we want, because we regard them as AAA. You yourself may hate that game, but that guy over there isn't as fussy. Or that guy over there, etc.
Hate to add to this wall of text, there's probably other points or indeed there are points against what I've said. I'll be interested to see what people make of this.