In countries with a high development index, the employment law usually prohibits 7 day long working weeks, and there is a provision of at least one day off (granted, since employment laws vary across jurisdictions, there is no universal rule).

The situation is different for small businesses and self-employed as they are the masters of their own fate (so to speak), e.g. if a cafe owner decides to run the cafe 7 days a week and work there in person themselves, that is their choice.

And then govt decided to stop for brownie points with church.

Sundays off are about workers rights, little to do with religion.

And you can still work on Sunday - in businesses that are allowed to be open (gastro, petrol stations, certain shops).

Why did they all happen to pick Sunday?

Weekend is only two days long, so it's not like you have a lot of options.

(Of course, my point is, Sunday is the day off, the last day of the week, and the "core" of the weekend, exactly because of the dominant religion.)