Because Sony focuses on exclusive games and word of mouth as it's primary mode of console advertising and MS just pays for advertising and exclusive "features". Word of mouth has pilloried MS this gen so maybe MS should change tactics.
MS rather sell you on something then have the quality speak for itself. That being said, Sony has also made a number of bad exclusives, but given that Sony is willing to produce so many they invariably create new IP gems like Uncharted and Last of Us, and a lot of great other games too like Infamous.
MS doesn't think games sell consoles as much as simply advertising/non-gaming content sells consoles. Remember MS spent $400,000,000 on exclusive NFL content for xone. That is not an extra zero. It's exclusive budget was 1 billion, first thing MS does is spend 40% of it on sports programming - a sport that few outside of the US even care about. That would have been A LOT of AAA new IPs.
Last gen didn't they spend like $150,000,000 advertising Kinect. MS is more willing to gamble on it's PR being effective than on making a great game.
Even already with PS4 and Xone, PS4 has a more diverse range of quality games. MS has Titanfall but people are just as happy playing the equally rated BF4 for their shooter needs, or COD. Ryse and DR3 and as crappy as Shadowfall and Knack, Forza is no better than the new Need For Speed as it's lacking content, muliplats are a wash but look better on PS4 BUT PS4 also has great games like Outlast and other quality indie titles, and now Infamous SS. So the console with the larger variety of games from various genres already is PS4.
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