Quite simply it's a competitor to the dollar, which is illegal within the US. You cannot issue currency in the US, to my knowledge.
https://en.wikipedia.org/wiki/List_of_community_currencies_i...
Of course, the feds are going to require taxes to be paid in USD, so you will need USD at some point.
The USA has already ruled that Bitcoin is NOT a currency...
I wish that they hadn't done that