Good question! We've been asking ourselves that exact same thing for a long time.
But here's our current thinking:
If we went with just selling keys then users who don't buy all the keys to a story at once would have to go back and make multiple trips through the Apple or Google stores. Especially if they wanted to read a story episode by episode.
One of Tapas' strengths (we hope) is that Tapas will always allow people to read, and pay for, stories at their own pace. You know all those books you buy but never quite finish? Well that should never really happen with Tapas because you can pay for a book episode by episode. But we don't think people want to go through Apple or Google every time they want to buy a new episode.
So with coins, you can basically have this nice little pot of currency that you can instantly use across all of Tapas. You can buy a few keys to a Fantasy book here, you can buy a few keys to a Romance comic there. And when you run out of keys you can quickly get more with the coins you already have in your account. We think this is better than having to go to Apple or Google and make a new purchase everytime you want to buy a key.
Another reason is that it's important for us to let users who may not have much money still be able to read premium stories through ad support.
If we only had keys we wouldn't know which specific keys those people wanted after viewing an ad. With coins, we simply award a certain amount after each ad watch. Then readers can use those coins to go get the keys they want.
And lastly, it's important to note that not all keys have the same value. Some keys cost more than others. For example, one $9.99 book may be longer than another $9.99 book. In that example the longer book would have more episodes but each of those episodes would cost less. That way both books could still hit the same $.9.99 price.