The reason is simple! We have been trying for decades to get the English language deemed as the official language of the U.S. , but to no avail.
The dimocrats fight it tooth and nail each time it comes up. If the vote were taken today, I'm afraid some repubs would be against it as well.
Sucks!
I wouldn't go live in Mexico and expect them to all speak English so I would learn Spanish. Likewise I expect that people from other countries learn English while living in this Country or go back from where they came from!