It's basically because USD is like the "world currency" everything is compared to. Businesses and companies working international almost always use USD. You know what Americans are like. They have to have everything and always be in control. The rest of the world just let's them think they in control. I mean they use the imperial system to measure things. That says alot already. I think after USD maybe eu and pound is next.
i think that the only reason can be that USD and bitcoin both are currencies, although currently the difference is that bitcoin is online currency while USD is fiat but still as both are currencies therefore we are comparing it with each other. i am sure that in future when bitcoin will become more popular and the users of bitcoin will increase then bitcoin will certainly overcome USD.