Um, what? That can't be "the" reason, but it certainly didn't help in the midst of a quagmire of other PR disasters.
- Weak effort to maintain customer loyalty - Literally the only meaningful thing MS did for Xbox 360 in the two years leading up to the launch of Xbox One was secure early Minecraft exclusivity. (Halo 4 seems to have been an all-around disappointment, otherwise I'd count it, too.) Meanwhile, Sony was breaking their necks to secure customer loyalty going into PS4 with excellent first-party software support, solid indie support, great PS+ programs, moving away from strategies that didn't work for PS3 early on, etc.
- Confirming people's worst suspicions - Microsoft pushed forward with DRM policies despite tremendous backlash even before they were officially announced. Backpedaling on those policies lifted some of the anxiety, but not enough to counteract Sony's basically flawless PR on the subject.
- Poor product vision - Microsoft's vision for Xbox One as an all-purpose entertainment box doesn't seem to have mapped at all to the desires of the vast majority of consumers. (Just like how it didn't really sell consoles last-gen.) Consumers were happy to get added support for TV/movie streaming in the boxes they already used to play video games (just like they're happy to stream content from their phones); they weren't looking to pay a hardware premium for a box designed around that multimedia functionality.
- Bad value - The price was a huge factor, especially since game consoles have been converging and becoming more like budget gaming PCs. No need to pay more for Kinect when all you wanted was a box that plays games.
- Games - Sony, having abandoned their hubris from last-gen, was much better able to position themselves as having all the relevant games. Not focusing on PS4 as an entertainment device also helped them position themselves as the company that cares more about gaming. Microsoft decided instead to go out guns blazing into the all-purpose media space (eyeing growth at the expense of an experience designed foremost around games), even incorporating their vision into the console's name.