They put too many obstacles between the customer and the checkout counter. The customer had to travel, potentially long distances. Then they had to wander the aisles looking for the product, compare it without any unbiased third party reviews. Then they had to travel back home. This all added friction, not to mention the overall price of the products.
All the opulence of those stores came from high operating costs, which were ultimately borne by the customer.
The sales staff expertise came with commission-based sales, which meant you could never really trust the salesperson because they had a vested interest in making a sale whether the product was good or not.
Mourning the loss of department stores is like bemoaning the loss of fancy horse carriages.