1) 25 years ago, supermarkets (with exception of Coop) didn't really sell appliances, books, video games consoles, etc. in the same quantity that they do now. So people don't need to go to a department store as frequently because of that, because they can pick up household. It's maybe not the best example, because it's been killed by the collapse of Philip Green's empire, but until recently my local Tesco had outlets for Topshop, Outfit, etc. in it. Sainsbury's has Argos in it in lots of stores now. And these are much more local to people's houses.
2) On top of that is online shopping. That's another place you can go that's more convenient than going to a department store. They have stock online! Often department stores did not.
3) When you do go to a department store, they're usually more expensive for the same or lower quality as elsewhere. John Lewis and Selfridges are the only ones I've purchased anything in in the last 10 years and that's because they have a good range of stuff, and for John Lewis in particular, it's known for having good customer service on things like returns of faulty goods and long warranties. It's also the only one of them that actually had stock in? Debenhams and BHS prior to closing were a mess for e.g.