It's probably because gamers expect a certain levl of triple (of course that expectation comes down often due to publishers), If after a year or two has passed and that standard isn't met then it's a wasted investment, so the company invests more in order to reach that quality, or surpass this.
If mid tier games actually sold well we wouldn't have this problem to such a degree.
For the big publishers (EA, Ubisoft, Acti), they couldn't wait because it meant that mid-tier publishers and developers wouldn't be able to follow. They raised the budget ceiling so high that it lead to a whole bunch of developers dying, and with them in control of the market.
Problem is, now this big budget arms race and gamer expectations have spiraled out of their control and it's affecting them too. And it's mostly their fault.
Which is why it's the gamers fault that mid-tier games sell so poorly, because they expect too much out of them. As a matter of fact, they're so much better in quality than most AAA games today (same with indie games & smaller games from XBLA/PSN/Steam like Double Dragon Neon & Bionic Commando: Rearmed).
They wanted AAA games, now they got them. And now that market is about to collapse even further this gen, especially if Microsoft can't get more Xbox One systems to sell.