Last time I checked, speakers and headphones are accessories. The exact ones that they've sold in their stores and are now not selling are accessories for Apple products. A Samsung phone or a Dell computer is not an accessory nor is it something someone walking into an Apple store would be looking for. There is a difference.And those other brands have mostly consisted of accessories (cables, cases, speakers, etc) that could be used with major products such as iPhone/iPad/Mac. That's why you don't see Samsung phones and Dell computers in Apple stores.
Now that Apple is likely elevating HomePod to a major product category, it makes no sense to have competing audio products in their stores.
The good news is Apple can do whatever they want as to how they choose to carry products in their stores. And don't need approval from others.