They had everything that you simply couldn't get anywhere else, a strategy that only an online shop could manage. Borders and other big chain had a similar-ish strategy, however they were hampered by their physical shops: they are forced to curate to fit floor space and, for a customer, delivery is not a positive outcome of going to a physical shop, so it does not matter how fast you can get something if it takes longer than the customer is willing to stay in the shop.
Amazon has continued to refine its online shop, but the key was to always offer as much as possible, even offering themselves as a platform to independent seller in order to vastly increase their catalogue. ( I have done business with amazon sellers that had sold only a handful of time per year, for a perishable product - that's ultra long tail )
There's an important paradigm shift here that needs to be considered. The idea of going to one place (what's a website again?) to find every variation ever made of every item you can possibly think of was completely foreign to people in 1993. In a nutshell, you can't GET to Amazon without going through Ebay.
So what would have happened had Sears' tried-and-true curation model come to dominate online shopping early on? It's hard to say obviously, but it still would have been ripe for disruption, from people who saw the online marketplace a bit differently.