My educated guess: internet shopping, combined with property owners that don’t want to face reality and tell their accountants their shops aren’t worth what they used to be (a 100% guess is that’s sometimes due to incorrect incentives. If a property manager gets a bonus based on the (perceived) value of a property, more so than on the income it generates, raising prices can be the optimal short-term strategy, even if it means shops close left and right)

I don’t think it will happen in the USA, but I think a solution would be to tolerate squatting, if the squatters improve the neighborhood.