I think one of the difference between stamp collecting and the demoscene is that while stamp collecting was quite big for a time, the demoscene has always been a niche thing.

For a time the demoscene looked bigger than it was, mostly because of all the cracks. But for the people who actually produced stuff or went to demoparties or even just seek out productions that weren't cracks, it has always been a small world, like a few thousands of active people worldwide.