It could be due to privacy settings. Some users may have set their accounts to private or restricted, which can limit who can be seen viewing your story. Also, Instagram's algorithm might not display all viewers at once for various reasons, like if there are a large number of viewers.
It might be that your account doesn't have the necessary permissions or that the feature to see all viewers is only available to certain types of users or under specific conditions. Check the platform's terms and settings for more info.
It could be that the feature to show all viewers isn't available for that particular story. Or, it might be a privacy setting that's preventing the full list from being displayed.
It could be that the platform has certain limitations on showing all viewers. Sometimes, it might also be due to technical issues or updates that are affecting the visibility.