It simply comes down to convenience. It may not be technically illegal to make cash payments for services or goods, but this would mean that you have to track a

It simply comes down to convenience. It may not be technically illegal to make cash payments for services or goods, but this would mean that you have to track all such payments and who the payee was, in case you paid more than $600 to a given entity. At which point there's enough paperwork already that cutting a check, or authorizing a funds transfer is easier. So while it wouldn't necessarily make cash transactions illegal, it would make them stand out more and be scrutinized more carefully.