Amazon was a real player for awhile, but they stopped treating developers well and their app store started to lag behind.
Back when the Amazon app store first started, they gave away a high quality free app every week. Great for user acquisition, then IIRC they stopped paying the app developers whose apps they were giving away, and the quality of the store overall started to drop off soon after.
If Amazon had made a play of taking a much smaller % of payment fees, sales and subs, they could have continued building momentum and been a serious competitor.
I presume they decided not to for some good reason, but the potential was there.