M&S are simply trying to fill their stores with products which actually sell at a rate fast enough for them to make a profit. That's why they've all but given up on trying to sell children's clothes in most of their stores and why they've removed men's suits from the rails in half of their shops:
https://www.bbc.co.uk/news/business-58374306
Elderly ladies might only buy one or two dresses or skirts in a year, plus just a few tops, whereas women in their 20s might buy twenty or thirty dresses or skirts, plus perhaps fifty tops, in a year. High Street shops are nearly all struggling to survive, so they're bound to concentrate on products with high volume sales (at the expense of those that don't sell in large numbers). BHS, Debenhams and lots of other fashion stores failed to learn that lesson and have simply fallen by the wayside.