Microsoft has reported soft game sales with their quarterly report. This is in contrast with Steam which recently reported that their last Summer sale was better than last year. Even the Xbox saw a decline in revenue.
Microsoft bundles everything into their Store app while games are usually handled by their respective publishers.
Steam is comparatively expensive which has lead publishers to developer their own apps to launch games and offer games. Many like Ubisoft still sell games on Steam but then load them into their own client.
Microsoft Games has chosen the idea that games will be offered everywhere including Steam as well as the Windows Store.
The decline in the console is more likely due to market saturation which means console sales are motivated more by price than features. The core crowd is also more interested in the upcoming console next year. For these reasons the Xbox is likely to be flat leading to the new console which will lead to an uptick in game sales.
Steam has been overrun with a vast number of games developed by independent developers. Many of these games are overpriced which leads to a lot of criticism in the Steam reviews.