Bottom line is USD is ultimately backed by force, by guns, if you live in the USA and you have wealth you need USD to stay out of prison, pretty powerful backing if you ask me, people tend to value their liberty a lot , now you can argue that initiation of force is unmoral all day long , but it is what it is.

Bitcoin is backed by market demand only, all of us promoting it because we believe in it's value as a currency, but this believe is not as rock solid as the power of the US government to force people to use the $