One guess is that the Forgotten Realms and Eberron supplement books didn't sell as well, compared to the generic D&D supplement books. The 3 books per setting a year strategy, may very well be a consequence of this.
It's not a guess, and it's not a big secret. I don't know
numbers, but yeah, setting books don't sell as well as "generic" RPG books. More importantly, every setting book after the first sells incrementally fewer copies.
Consider:
For a book like
Player's Handbook 3, the potential target market is the entire D&D-buying public. Sure, not all of them are going to buy it, but they're the
potential market.
But for (just to pick a setting out of a hat) the Eberron core books, the potential market is much smaller: It's the subset of the D&D-buying public who are interested in Eberron.
Then, for every
subsequent Eberron book, the potential market is even smaller. For (again, just picking an example out of a hat) a book on Sarolona, it's the subset of the Eberron market who are interested in Sarlona.
And so on, and so forth. It very quickly reaches a point where the resources being pumped into a setting book could instead be pumped into a "generic" D&D book that's almost guaranteed to sell better--sometimes by an order of magnitude.