The field and range of model railway products has expanded dramatically over the past few decades. For a bricks and mortar only model shop to survive, it needs to hold a ready to supply, stock of all the products it knows it can sell to a walk-in customer. That implies (actually requires) a massively larger inventory to store and finance than it needed in 1970. Just look at the livery and number/name options of even the simplest big two RTR products. The only way to start a B&M model shop today is with a huge monetary investment.
Hence the rise of build to order technology, only sold direct, from a single central facility, or near monopoly, huge on-line suppliers, or smaller on-line specialists suppliers with a unique, legally or economically, protected product line. They are the only economically viable solutions now.
Tim