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.
Yes I agree with you that most of the world economies are dealing in US dollars so it become a common practice that we see and covert the value of different commodities to US dollars and this is not the case with only with bitcoin. By the way I always convert/google bitcoin to my local currency after seeing its value in US dollars.