There is a particular game which is in beta release in on country, in my country it is possible to register on a waiting list and in South America it just says "not available in your country". Is there a general explanation why the availability is different between different countries? Is it for marketing and purchase power adjustments or some other reason?
Google for reasons not made public has a list of countries where app and Google Content is available Country availability for Google Play apps & digital content.
Developers may choose targeted release for their own reasons. Google guidance on this - Distribute app releases to specific countries - Play Console Help. I am not a developer but I guess reasons could include localization, scalability of app, revenue expectations, taxation issues, political reasons (Indian ban on Chinese apps recently comes to mind), reviews (I am aware of a famous developer who doesn't release his apps in some countries because of unfair reviews).