Well, it wasn't added to the rules until 1906, and traditionally, we didn't have a national language, because the framers didn't see a need, and the colonies at the time housed people from many European descents (especially since colonies such as New York weren't originally British, many portions of the newly formed country were very recently French, lost during the French-Indian War, and disputed with Spain, like western Georgia). Later on, it became increasingly relevant as the US "acquired" territories with predominantly non-English speakers (everything west of the Mississippi) through either Conquest (Mexico, Hawaii) or purchase/treaty (France - The entire midwest and Louisiana, Spain - Florida).
Teddy Roosevelt was the first major person I know of to champion the English-only movement, in the early 20th century, and several states outright rejected it, having de jure multi-lingual government document requirements, and Louisiana having 2 de jure official languages (English, French).
I don't really see what the benefit of enforcing a national language is. I don't see a multi-lingual government as a shortcoming, I guess; and a majority of the continental US is in fact land that was acquired while populated with Spanish and French speakers. I say let the language demographics evolve naturally.
