Also, Essentials hit the shelves late in Q3, but there was a run-up to it that was not exactly well-received - starting with a controversial update to the CB in June, that, along with previews, created some negative impressions of the new release.
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That still doesn't explain how the APG was able to match sales with the entire Dark Sun project plus the Psionics splat, since none of those involved Essentials. (And keep in mind that a tie in this context was a disaster for WotC, given the relative size and resources of the two companies.)
My point is that 4E was already suffering from book sales problems prior to the release of Essentials. And that doesn't seem like much of a mystery to me. By the summer of 2010, they'd already released a mountain of splats and settings, and there was almost certainly a diminished rate of return on new splats. Historically, that's always been the case. Second, by early 2010, a lot of people had probably realized that, for a fraction of the price of one splatbook, they could simply pay for a one-month download and get all the updates plus the errata, on the DDI character builder. In other words, 4E was already an old man less than three years into its release.
I think that's exactly why they've already indicated they'll be producing materials at a slower rate for 5E, at least when it comes to splats.