The "Upgrade from XP to Vista" rollout backfired for a ton of reasons, but if I had to summarize it was due to these facts:

1. Microsoft did not work closely enough with device manufacturers to ensure device compatibility from Windows XP to Vista. Printers and video cards primarily.

2. Consumers now had to choose between 6 versions of Vista instead of "Pro" or "Home", which was confusing.

3. x64 architecture was introduced, which doubled the number of possible choices consumers had to make and inevitably lead to some consumers being very disappointed with their purchase.

4. Due to the introduction of Aero and some poor quality control on the part of the Windows team, Vista required vastly more resources than what many Windows XP machines were capable of delivering. Vista's minimum install footprint is something monstrous like 16-24GB, which was a stretch for Windows XP machines manufactured as long as 6 years prior. Since then, each version of Windows has required less resources than the edition before it. Sinofsky emphasized that.

So in short, I think Microsoft realized that the QA burden is a lot greater than they previously thought and have left it up to their most technical consumers to decide whether or not to upgrade, relying instead on hardware refresh cycles to do most of the heavy lifting for the bulk of their consumers.

Unfortunately with the advent of the "multi-device consumer," those upgrade cycles can often be much longer than three years now.

Despite those concerns, Microsoft sold copies of Windows 8 for something phenomenally cheap initially, $49 IIRC, in hopes of getting people to upgrade their existing hardware. I believe they discontinued that practice 6 months after the initial release of Windows 8.