It's easy for Americans to learn Spanish. 

There are millions (~61 million) of Spanish speakers in the US which means it's easy to practice.  There are some places where Spanish is primarily spoken around town -- Miami for example.

Many English words are borrowed from Spanish. 

Learning to read in Spanish is much easier than say Korean, Chinese, or Japanese since the alphabet is almost the same.

Furthermore, it's easy to travel to Spanish speaking countries like Mexico, Guatamala, Costa Rica, Colombia, or Peru.

According to the Defense Language Institute, Spanish is a Category 1 Language which means it takes native English speakers about 23-24 weeks (575-600 class hours) to learn to proficiency. 

Compare that to Category 4 Languages - which take 88 weeks (2200 class hours plus a year of in country study) to learn - and you will will understand why many Americans speak Spanish.

Its because spanish is the native lenguage of  many coutries in South and Central America, moreover, many people from Spain have exilied to America (cause of Spanish Civil War). Its the reason of why in South of U.S so much people speak in spanish, and north way still having more % of people who speak in english. (:

PS: Please correct me in something if im wrong, thanks. 

Puerto Rico is part of the USA. People from everywhere in the Americas come to the USA thinking it is an economic wonderland. Many of those who come to the USA plan to return to their cultural homelands so they want to preserve their language. Some have skills for earning a living and see no benefit in learning English. Others learn Spanish in order to communicate with their Spanish speaking coworkers or Spanish speaking employees. 

I think,because there are some people from Central America and South America.moreover is very closer to Mexico.