I suspect that since there is at least one species of wild garter across all of the lower 48 states, most people would rather catch their own than buy from someone else. Also, without doing research on the internet, most people are unaware of the huge differences between each garter species. Most people have no desire or motivation to learn more about such common snakes, so their variation goes unnoticed. Yet, Europeans appreciate them more because they're exotic, and they have to do the research to find which of the many varieties they would like to buy. Maybe European snake owners are a bit more educated about garters before they choose their first. Whereas, most 10 year old kids have probably caught and kept one in the U.S. I think that if more people were aware of the variety of garters beyond their backyard, they would be more interested in them.

Personally, I think garters are much easier to keep. For one reason, they're smaller, so a person can house more individuals in the same amount of space. Maybe the size thing is an issue? Maybe people want a large snake that impresses their friends? Who knows? Their activity levels, striking colors, personality, diverse diet, cost, friendliness, and availability are all major selling points for me. Personally, I don't want a snake large enough to harm (even cut) one of my children.