Before the days of the internet there was little competition for the local stamp shops which meant they were able to charge higher prices, but in return had a lot smaller audience from which to make their profit.
Now you can buy stamps from anywhere in the world easily, the competition has brought prices down. When I want to buy a particular stamp to add to my collection, an internet search will probably reveal several places where I can buy it so the ones charging 50% of catalogue price will not get my business.
Simple economics!