[Crosseye_Jack]

I would say it is down to AWS integration mainly, give an easy integration path to AWS services and dev's might be more willing to use AWS and that's where they make the money back.

But Twitch (an amazon company) also had its own game store they were pushing (and giving a cut of any sales referred from live streams to the streamer) but Twitch closed that down last month (But they still have free games / loot which is linked to your twitch account and accessible via their desktop client). So it is possible they might of pursued that route more heavily and did what Epic has done with their store (lower fee's when using their store and engine over using Steam and their engine).