Because of high rents they have to pay?
Because of competition caused by e-commerce?
Because of the gap between rich and poor?
The impact by the gap between rich and poor?

My reasoning: common people have less and less money in their pockets which they could spend at little and midsize shops. The whole economy still makes very much profit, but the gained money is not in the pockets of the common people but lies on the accounts of a few extremely rich people which don't know what to do with their giant fortunes; they certainly don't buy the goods of small and midsize shops, and therefore many small and midsize shops go bankrupt.

Maybe, we have a kind of spiritual problem: the "heresy" of Adam Smith.
https://en.wikipedia.org/wiki/Adam_Smith