Mixture of things: cost, convenience, variety. You gotta understand that the majority of dept stores in Britain offered high prices, poor quality, and low variety. They were also inconvenient as you couldn't park your car in the lot outside the store. British authority has this obsession of trying to keep your car as inconvenient to you as possible to force you onto public transport. But public aren't having that so since the 90s the rise of the retail park has occurred.

Retail parks offer parking outside like strip malls and stores offer cheaper prices than the likes of BHS. Basically as somebody else said supermarkets and the new dept stores. Also, variety online means you can finally find what you want and not have to make do with the near equivalent in model at a high price in a dept store.

A lot of these dept stores only existed in Britain due to the limited selection. They'd have died a long time ago if the internet was a thing back then. I didn't ever shop in BHS so can't say I'd ever see the loss myself.

Even if people had free time, why would they want to spend it in a store if they can get the same result without having to travel to the store and spending time in the store?

Obviously, many people enjoy shopping, but even they will probably reduce the time they spend in a store and/or spread their purchases between in store and online, so the physical stores have to resize for the new demand.