They lose revenue and the stores start to look dated and because they have to pay rent they cant compete with a big warehouse. Eventually everything looks dated and costs more so people stop going.
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.
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.