It could be that the stories don't meet the content guidelines of those platforms. Or, the platform simply hasn't made a deal with the rights holders to offer those stories.
Maybe it's due to licensing issues. Some platforms might not have the rights to host certain stories.
Often, it's because the platform doesn't have the technical infrastructure or resources to handle and present those particular stories. Another reason could be that the stories are too niche and don't have enough potential audience for the platform to invest in making them available.