Kannik
Legend
Following on this and Snarf's additions, I concur that the Hasbro dictates ($1B or DIE, essentially) likely played an oversized role on so many things that they ended up compounding in a not-great feedback loop (like how the choice to have no SRD or license tainted the views of some or even turned them away sight unseen). 3.5 came out a mere 3 years after 3.0 (which itself started some resentment of edition churn), and with an even greater push for $$ with the $1B dictate, a drop of any number compared to the previous edition would be seen as a problem. (I have my inklings and educated guesses what that drop actually was, and as with Snarf I'd love to see the actual numbers, but again it's probably immaterial from Hasbro's point, since any devaluation would be in the wrong direction of the BIG BRANDS, NO WHAMMIES mantra.) (I probably just dated myself there, didn't I?I'm pretty sure it came down to the fact that 1), D&D wasn't making all of the money. There was actually a serious rival in the form of Paizo, who built their success on the grave of the previous edition. And 2), not all of the promises of 4e came to fruition.

On the whole 5e seems like it was a low-risk move for WotC and Hasbro to pursue. (So much so that it 'had' to be done.) Fortunately, at least from what I understand, the $1B thing had been dropped as a death line march, and so they only need to grow things, not reach for unatainable heights. And since they could be confident that most who went from 3e to 4e wouldn't bail as they had shown themselves to be open and expansive, there was that chance with the new edition to recapture some players who had gone elsewhere, either due to 4e itself or due to that myriad of other elements that formed the feedback loop noted above. And then there was also the usual new edition buzz that could entice new players, even to those not already playing. Getting bigger number seemed almost inevitable.
And then, as we saw, with the magic timing of actual plays, the Marvel effect allowing formerly "geeky" things to be seen as "cool", some TV show inclusions, and technology that had caught up to some of 4e's ambitions, etc and etc, things really got turbocharged and 5e is where it is today.