Because Bill Gates is too conscious of the money in his pockets to actually invest in XBOX so it finally pulls ahead instead of tag behind PS like it always has been since the rivalry between the two started.
Bill Gates has noting to do with Microsoft. It was a means to an end for him and he washed his hands clean of Microsoft years ago.
OT: it sarted at launch when they introduced some shady practices behind their “pay to play” gimmick. You were going to be required to be online at all times for any game meaning you were forced to buy Xbox life to play any and all games. They were going to enact digital copies only no discs, among other things. Plus the hardware, despite it superior power, was increasingly difficult for devs to utilize their gaming engines on forcing many of them to rebuild their engines from the ground up whereas PS4 operated more like a PC.
Many fans were pissed by this and jumped to Sony. They were even booed at the E3 Showing for it, retracted and did away with this, but the damage was done.
Sony, on the other hand, opened up too the indie market and dominated the market with AAA PS exclusives which hit XBox right in the wallet.