There are apps in which are not been received from a trusted credential and such could poses the kind of harm they wouldn't allow to have on their list of supported apps, so they will require then to go through some required procedures just to secure the the safety of their users, apps store doesn't support an app is not a close deal that the app in question is not trusted or is having harm in disguise, but any non compliance to set protocols may only lead to being barred from a service, which should tell more on why some apps were not found on the apps store.