This right here.
Developing for iOS is easier, because there are very few differentiations in aspect ratio and resolution. With Android it seems like every new device is a little different, and if you look around at different apps in the Android store, you'll see people with this phone or that tablet having issues with an app, forcing the developer to debug their apps for specific devices.
Also, there are more iOS devices than Android, so it's a larger market.
Finally, I wouldn't be surprised if Amazon is looking at non-Kindle Android tablets as competition, and thus much more hesitant to develop for them. When you look at Kindle vs iPad, it's a whole different device & pricepoint, so while they are somewhat in competition, in many ways they're really not. It's like KIA viewing BMW as competition. While they both sell cars, they're appealing to very different people. Android tablets, however, tend to be more budget friendly, and in direct competition with the Kindle Fire. So that leaves Amazon in a quandry: should they offer a Prime Video app on Android and increase the appeal of getting a Prime membership, but potentially lose a lot of Fire sales (and thus possibly lose other sales that would've been made by keeping users locked into the Amazon ecosystem). Obviously for users, the more accessible the services are, the better for us, but Amazon has to look at things in terms of profits, both short and long term.