Well yes, if your theme is so good why don't you sell it on the Theme Store?
The answer is simple: Shopify imposes functionality limits, design standards, and a limited business model, which restricts us in what we want to offer with FullStack.
To get rid of all its limits, and keep our independence with the subscription model, you made the choice not to publish FullStack on the Shopify Theme Store.
If Shopify decides to offer more freedom on its platform in the future we will be happy to publish it.