I think the supply and demand aspect of the conversation has its place. However, the question asked by the JacKaL is "why should we care that only a tiny percentage of the USA population is doing bonsai?" Supply and demand and/or the affects of it aren’t going to make us care any more or any less.
Here's where I'm coming from with the whole supply/demand thing. I used to live in Chicago 20 years ago. At the time, there were TWO bonsai vendors in the entire Chicago Metro area. One was about 20 minutes away. One was about 30-40 minutes away. I used to travel to San Francisco to buy trees, Chicago not being known for its mountainous regions or craggy yamadori. It was not all completely bleak - Chicago has the Chicago Botanic Garden (with one of the best bonsai collections in the country) and the Midwest Bonsai Society. But if you live downtown, or in the western suburbs, that 45 minute to 1:30 drive to Highland Park for a meeting can get somewhat tedious.
Now that I leave in Southern California there is a BIG DIFFERENCE in the ability to go to nurseries, purchase supplies, join clubs, go to shows, etc. It is night and day. I have a study group of 6 talented people right in my area. There are a number of nurseries we can go to, retailers with rooms full of bonsai pots and tools, as well as garden centers that have supplies for bonsai as well as related horticultural interests (orchids, Japanese gardens, koi ponds, etc). There are several top quality shows in the area every year, as well as bonsai professionals who have been in the hobby for decades, if not generations.
Now I don't believe that the inherent interest in bonsai is any higher in Southern California than the Midwest, with the exception of a stronger Asian cultural presence. I just think it is EASIER and CHEAPER to practice bonsai here. Higher demand begets higher supply and more price competition. Additionally, higher demand creates ENOUGH supply that retailers can sell at a lower margin and still make enough money to live. If one day half of the bonsai enthusiasts in Southern California would vanish, I would feel the impact. Stores would close. Nurseries would shut down. Shows would be canceled.
Certainly if all you do is practice bonsai in your backyard with yamadori, your own handmade pots, and tools from Home Depot, you probably won't notice a difference. But for the rest of us who are involved in the bonsai community at large, I feel the art is either growing, or it is dying out, and I try to support it in small ways, here and there.