For the same reason that Argentinian, Mexican, Colombian, Canadian, Ghanaian, Singaporean, Australian, Ivorycoastian, Nigerian, Brazilian, etc are not languages, but dialects.

Regardless of what Max Weinreich may have said (which was more of a political/philosophical statement than an academic one, and he didn't even coin the phrase), the variation between english dialects is not enough to call it a language.

Some pidgins are considered languages: they're spoken as lingua franca by large populations and is quite different from English.