In my personal experience most people think of cryptocurrencies as a Ponzi scheme, some kind of money laundering operation, a tremendous waste of energy, or some combination of those three. In light of that, I think a cryptocurrency ban would be easy for politicians to sell to the public using those terms.
How effective the ban would be is another question, but I am mainly talking about how I think the politicians would justify the ban.
Oh, I didn't realize you only meant how easy it would be to pass the ban. Of course it would be easy to pass a federal ban in the United States, where it's easy to pass many things even when they're extremely unpopular. Marijuana is still banned federally despite well over two thirds of the country opposing the ban!
How effective the ban would be is another question, but I am mainly talking about how I think the politicians would justify the ban.