It's selling well for a number of reasons.

For starters, a few years ago, when Microsoft stopped their focus on 'core' games and original IPs, and instead started to focus on Kinect and dashboard apps, they pretty much lost a lot of fans. First few years of 360 were excellent. Last few years: Not so much.

Where Microsoft completely dropped the ball in that period, Sony picked it up big-time, especially with their PS+ program, as keeping that focus on bringing new original IPs.

TBH, by the time next-gen was about to be announced, Sony was already the darling (again) of the gaming-community, while Microsoft had messed up big time.

And then Sony announced their new console, and Microsoft followed, and Sony simply had the better specs, the better price, and Microsoft rubbed the gaming community the wrong way AGAIN by announcing their anti-consumer practices, like the 'no used sales' policy, the 'log in once every day' policy (or 'always online' policy), and the mandatory Kinect 2.0.

By then, the battle was already over and decided: Gamers would massively flock to Sony, while Microsoft had to do everything it could to backpaddle on some (most) of their intentions and trying to win back some fans. Which they did, to some extent, which is to be applauded and admirable, but of course, the damage was already done.

Right now, facts are that Sony has a much better recent track record when it comes to listening to what gamers want, bringing them diverse and original new experiences, and obviously they're selling a console with better specs at a lower price than Microsoft, although recently Microsoft has been offering good-value bundles which dropped the Kinect and included games*, so you could argue they're now pretty much priced similar. Which is obviously great for consumers.

( *= Sadly downloadable games only, instead of retail-discs, which for me is a reason to not consider buying an XBO yet)

Unless Sony now does something extremely stupid and completely messes things up, I don't really see them losing this huge lead.