Well, back then, the stores were required by law to close at 1 PM on Saturday. There was a LONG Saturday once a month where they could stay open to 4 PM once a month on Saturday.
These laws were intended to prevent the over-work and abuse of common workers, and contribute to the european way of family life. The laws pretty much did that.
The basic worker at a store was a minimum wage employee. Even with all the long hours stores are open these days, the management just keeps the hours down of the employees and hires more minimum wage employees to spread out the hours. No one gets rich in Germany on minimum wage.
What caused the stores to stay open to almost American-like convenient hours was from two sources. CONSUMERS got lazy and wanted everything EASY for them. In USA, almost no one wants to cook their own meals from fresh ingredients. That takes too long and is too difficult for many (hard to believe, but that is the syndrome) So, consumers demanded more open hours.
The other push was from big business. They saw an opportunity to become real huge real fast if they could market the best mix of convenience and variety, plus stay open real late. They grew real fast and squeezed whole sectors of the population. They hired more workers at dirt cheep wages, but they never made enough to do anything but to keep living with mom and help out a little with electric bill.
So basically, it was laziness of consumers and greed of big companies that got us to what we got.
There could have been a better balance of convenience and maintain the small guy's shop, and the country's basic way of life, but we got what we got. Now the small guy has got to work 10X harder to compete, it isn't easy. He literally has to spend all day from 0600 to 2300 in the store to have any chance of keeping up. That isn't sustainable without any larger success. That is the way of business yes, but the cause.effect on life is big. All this greatly changed the European mentality and way of life almost like a wildfire. European way of life did not change entirely and there are still some good aspects relatively unchanged, but all in all, the wave of convenient consumerism and big company greed had a cost that wasn't worth the added convenience it fostered.