Well, there isn't enough demand to put a hobby electronics shop in every mall and on every major highway. In fact it's a mystery to me how Radio Shack got as big as it is, other than it predated big box consumer electronics store.
What you need to support a bricks and mortar store network like this is an answer to these two questions:
(1) Why will people go to the store?
(2) What will they end up buying when they get there?
Have you noticed how bookstores tend to have coffee bars in them now? It's because you're thinking about going to Starbucks for coffee, so why not go to the one in Barnes and Nobles and do a little browsing while you're there? Granted, you may go there specifically for books some time, but having a coffee bar gets you in the door enough more times that you end up spending more money there annually than you would otherwise.
If you're going to buy a phone, why go to Radio Shack instead of your carrier's store? If you're going to buy a radio or a set of speakers, why go to Radio Shack instead of a big box electronics retailer? About the only reason I can think of to go to Radio Shack is if I needed an odd sized battery, which is not such a bad draw but it wouldn't draw me in more than once or twice a year.
Sure, if Radio Shack had a great parts counter it might get people like you or me to go there, and we might walk out with a headset or a cell phone. But there aren't enough people like you or me to put a Radio Shack in every mall and along most major highways. If they could just get enough people in the doors, they could sell them all kinds of electronics-y stuff, but there's nothing that will bring lots of people in the door. Every time I go to Radio Shack, it seems like the number of customers is something like 1.2x the number of staff. That's no way to make money.