There were several obstacles to using SR, each of which would have been difficult or frightening to overcome when the alternative is doing transactions in person with cash. You needed to understand TOR (which, while not especially difficult to use in general, is harder than opening Chrome and going to Amazon), you needed to understand and obtain BTC (either mining them or purchasing them, which was generally somewhat difficult given that you couldn't use credit cards or paypal or other simple payment methods), and then you had to figure out PGP for sending your information to the vendor.
It's all definitely doable and none of it really requires much technical knowledge but there's definitely some friction that I assume would intimidate a fair amount of people.
