It's call making an early investment. There are new revenue models and independent developers being brought to mobile and PC because of less restrictive policies for game submissions. That means catering to indies who can or will be future AAA developers, that means having a larger variety of games that are big and small, and having a larger audience who can learn to appreciate all different kinds of games.
This part of the story isn't exactly new. MS has always partnered with indies that they've deemed talented enough to provide a publishing agreement (and extra dosh, of course). It was restrictive, but considering how Sony sidelined a few devs who would eventually win big at IDF and other events, and considering how it was MS' Summer of Arcades that brought "indies" to critical mass, I think the latter company has had a good rational for its marketplace curation MO up till now.
By any indication of Gamescom, Sony's courting the b-listers (sans a few exceptions) instead of the indie devs that are actually AAA potential.
Everyone else just jumped on because, hey, more freedom!