Actually, there are several possible reasons:
- Not available in the user's country
- The AppStore take some time to deliver an app update to everyone (should be available within 24 hours)
- Not available for the user's iOS version (a lot of apps are built for iOS version up to x.x)
- Not available for the user's device (32bits, iPad or iPhone..)
- Phased Releases option used in AppStore Connect
This list is certainly not exhaustive*
- How to browse iPhone App on iPad:
http://osxdaily.com/2017/10/18/download-iphone-apps-onto-ipad/
- 32 bits apps aren't showing up in the AppStore anymore:
https://www.gizmodo.com.au/2017/06/32-bit-apps-arent-showing-up-in-the-app-store-but-dont-panic-yet/