I think because crypto currency became really popular and is completely untracable (impossible to identify scammers). For that reason it is used a lot for illegal transactions (eg. guns, drugs, even assassinations and some really weird stuff).
They're as you said, scams. They're popular because most people don't understand cryptocurrency. Times that with the people who want a "get rich quick" scheme and you get the abundance of these scams.
It's probably easy to do and irreversible, with no real safeguards. Like people said, those who are looking to get rich quick are more likely to fall for these scams. Crosses over with people who are interested in crypto