Earlier quoted context omitted.
The problem isn't the product. I was going to compete with Steam at one point and we had objectively better systems and a better client. The problem was the critical mass issue - no users, no publishers, neither want to join without the other. Amazon will definitely get publishers but will users join? That's not a given.
Amazon already has customers. If their other products are to go by, they'll just give you an account if you have an Amazon account. Probably combined with free games if you have a prime account and you can imagine that it won't take much to compete, at least not for a company like Amazon.
That was my thought. They already give away free games over Prime, if they leverage that they have already given a large number of people stake in their new market place. Plus they own Twitch, I don't believe there is a publisher who isn't interested in the idea of people being able to impulse buy whatever there favorite streamer is playing without even leaving the stream. The strategy is pretty easy actually, give streamers a cut of each sale and encourage them to put up notifications when it happens like they do subs and cheers.