Why? Cause shops are open on sunday? Having no workers rights makes that a lot easier
Yes. Shops being closed on Sundays is a major PITA. I have 2 days off a week. So I have to buy groceries in overcrowded shops in the evening or in overcrowded shops on Saturdays. Or I drive across the border and buy in Luxemburg, on Sundays. So the VAT I am creating stays in another country. Which is just plain stupid.
Also: workers' rights and shops being open on Sundays aren't mutually exclusive.
The churches don't have enough political influence to keep Sunday a rest day. That we still have a mostly closed down Sunday (minus vital and emergency services and recreation) is union influence. IG Metall and Ver.di would skin the SPD alive if they were to propose abolishing it.
Consider the alternative: All your friends have different days off, so organising a grill party becomes a once in a summer opportunity when all your days off happen to align.
On the contrary there's a lot of shift work in industry, especially IG Metall's "core" clientele, metalworkers. A blast furnace don't care whether it's Sunday you need workers to work it, 24/7 -- with extra extra pay for night shifts and Sundays. But IG Metall also covers the engineering side and with that IT workers, plenty of white-collar jobs included it's a really big tent.
Shops closing on Sundays in Germany is no workers rights issue. No one is asking workers to work 7 days a week.
Germany as plenty of students, for example, who'd love to have a job on the weekend because they have the freedom to choose a bit better when they work and when not.
The reason Sunday to this day is still a day when almost all shops have to close is mostly religious. There are restaurants and some other shops that are allowed to stay open and most of them choose either a different rest day or make sure that they have someone on any of those days. One workday on a Sunday is plenty to fill out a typical untaxed low payment job that are very useful to students and others looking to just get a bit of an income.
Actual workers rights aren't telling people that they can never work on Sundays, they're guaranteeing people that they will never need to work too much.