US Dollar is going down in price, US economy is collapsing. There are many other countries which has more Bitcoin users, like Russia, China, Japan, Philippines and the whole Eurozone. Even ICOs are forbidden in the USA. It's not a so Bitcoin-friendly country, its national currency is not so strong. So why price Bitcoin in $?
There could be many other more appropriate currencies like EUR, CNY or RUB...
Its not only BTC to USD, almost all currencies are compared against USD because USD is the world’s primary reserve currency, a lot of countries hold reserves of the USD.