By cash do you mean physical money, like coins and notes, or liquid amounts in a bank account? The problem is physhcal money is not a good way to move large amounts around, and most bank accounts have a great deal of oversight.

No-one cares about $75 being used to buy heroin on The Silk Road, but people will notice $75,000,000 being moved - and Bitcoin is not anonymous either, merely psuedonominous.