A story might be unavailable on a particular platform because of licensing restrictions. Maybe the platform couldn't come to an agreement with the rights holders. Also, if the story contains content that violates the platform's terms of service, it won't be accessible there.
There could be a few reasons. Sometimes the story doesn't meet the platform's quality standards. Another possibility is that there are legal issues preventing its availability. Or perhaps the author or publisher didn't choose to make it available on that specific platform.