Why do many U.S. immigrants learn English?
English is the most common language in the United States (80% of the population speaks English at home). Additionally, all formal education in the United States (foreign language classes notwithstanding) is conducted in English. It is worth noting however that the United States (federally) does not have an official language (according to the last census over 350 languages are spoken in the United States).
To someone who immigrates to the United States this presents a particular challenge, a language barrier. Knowledge of Basic English is almost a prerequisite, for example if you go to the grocery store, need to catch a cab, ride the bus, drive, maintain a job, et cetera an overwhelming majority of communication will be conducted in English. Essentially if one doesn't have at least a fundamental understanding of English it will be very difficult if not impossible to maintain a decent quality of life.
Immigrants from Spanish speaking countries have a slight advantage over their non-Spanish speaking peers. Spanish is second-most commonly spoken language in the United States (about 13% of American citizens speak Spanish at home). Unfortunately this doesn't assuage much since a native Spanish speaker will undoubtedly come across people who do not speak Spanish.
All this to say, English provides some commonality in an otherwise extremely culturally and linguistically diverse nation.
I hope this helps!