What do you guys think is the reason?

Games.

A horrible reveal and lackluster first party games.

same reason psp sold less than the ds

Their vision at launch. They were so focused on TV that they forgot it was a games console first. The lack of power, Kinect costing the launch console to cost $100 more than ps4, always online nonsense and Don Mattrick in general.

The main one is not being a playstation, this basically resumes the other reasons.

It's called an Xbox and not a Playstation.

Plus awful first party game output, and bad marketing and awful first party game output.

But with everything being equal PS still sells more because of it's brand.

The fact that it failed to connect with the Japanese market has to be a reason, right? I've heard that Microsoft has always kind of struggled with selling its hardware in Japan but like, Nintendo and Sony consistently do well in that market (how shocking that a Japanese company would do well in Japan of all countries) and for some reason Microsoft just cannot do well there. Also the 3rd party support is kind of lackluster and I feel bad for saying it but at least with the 3rd party companies I follow they seem to favor the PS4 and Switch when it comes to actively supporting those systems

As an Xbox One owner, I would say their mistake was to force Kinect which meant higher price and the whole deal about always connected. If Xbox had started with the vision it has now of having different models, Game Pass as an optioin, and with equal price range as PS4 they would have done way better. I personally like how I can play games on my Xbox and continue on PC when the wife wants to take over the TV. That should become a standard on all games.

There is a laundry list of reasons, but here are some big ones.
1. The reveal. The reveal and initial features of the Xbox One were an unmitigated disaster. Even some big Xbox fans were disgusted and vowed to jump ship to PS4.
2. Price. The Kinect 2.0 was initially bundled with all Xbox One consoles. A lot of people had no interest. You were paying $500 for a console that was less capable than the PS4.
3. Lack of true exclusives. Very few games are truly exclusive to the Xbox One. And more importantly, the first-party output they had were not received as well as the Xbox 360 library.
4. Lack of Japanese titles and exclusives. I know the Xbox consoles are a non-starter in Japan. But even the Xbox 360 managed 1.66 million units. The Xbox One has only sold 0.12 million units in Japan. It'll be lucky to reach 0.15 million in Japan before it's discontinued.