Because over the past few years Sony has released very high caliber exclusives. For example Bloodborne, God of War, Horizon Zero Dawn among many others. They have stayed very true to the brand and focused on developing very high quality games rather than generic fps and sports games. The hype behind the games have also lead them to be console sellers and the marketing has overshadowed Xbox. While Sony was releasing God of War, a game that received 10/10 from many review sites, Xbox released Sea of Thieves, an unfinished repetitive multiplayer game that could be finished in a few hours. 
Sony has made the right choices leading to its huge lead over xbox.