Most other sports (besides cycling) don't have "speciality" stores for people to buy sports specific gear, so how did running-specific stores come about?
You must not get out much.
Your last part is an interesting question. My guess is that at some point someone realized that runners were not getting properly taken care of at Sears or wherever people were getting their running gear (such as it was). As running boomed in the 70s following Shorter's win in Munich and the publication of books like Aerobics by Ken Cooper and Jim Fixx's book "The Complete Book of Running" (although it came out in 1977), more shoes became available and people saw a way to make some money.
I remember buying my first running shoes in 1977 (Nike Waffle Trainers and later LDVs) from the local sporting goods and hardware store. The running shoes were right there with football cleats and Chuck Taylors.
Yep, then I bought the Internationalist and next Internationalist at real running stores.
Now most recently, I bought the Nike Winflo 5 at Discount Shoe Warehouse, next to all the dress shoes and purses.