Because it got popular real fast in Russia on instagram, while it was popular in America already. Now it's worldwide, last night I had a check up by a female police and she was wearing a charge hr while I had a surge on.
Other than that, if you train on a high level, you need heartbeats. Every heartzone has it's own purpose and it is important to apply it, not many people know about it. That's why it's so useful.