Prior to the 1994 Act shops were allowed to open on Sundays but the items that could be sold were restricted. This was ok in the era of smaller more specialist shops but it didn't really work with larger shops that sold a wider range of goods. One famously quoted example was it was legal to but a 'top shelf' magazine but not a bible.
In the early 1990s some of the large retailers started to flout the law and open on Sundays. This caused a considerable amount if uproar and there was a large 'Keep Sunday Special' campaign. Some retailers were prosecuted for opening on Sundays but I seem to remember the penalties for doing so were so small it was worth their while just paying them and continuing to open.
There was much concern about the viability of small shops if large retailers were allowed to open unrestricted hours on Sundays, as well as the overall impact on the fabric of the country. The eventual compromise was the Sunday Trading Act 1994 which allowed large shops to open for only 6 hours. The penalties for breaking the law are large and in over 25 years so large retailer has sought to break ranks.
It's worth pointing out that Sunday trading doesn't just impact shops and their staff. You also need deliveries so distribution centres and their suppliers need to work, you need better public transport for workers and customers, parking attendants, street cleaners, cash in transit collections, security guards, retail support centres and field management support. The list could go on.
Overall we probably have the balance about right. I certainly wouldn't want to see Sunday trading hours increase, it would end up with Sundays being no different to any other day.