Why do you think these countries speak Spanish and not other languages?

Spanish is a part of the Ibero-Romance group of languages, which evolved from several dialects of Vulgar Latin in Iberia after the collapse of the Western Roman Empire in the 5th century. As a nationality, "Spanish" refers to people from Spain and only Spain. People who are from Mexico are, well, Mexican; people from Guatemala are Guatemalan; and so on. I won't try to settle here any controversy over how to use terms such as "Hispanic" and "Latino." Suffice it to say that traditionally in Spanish, hispano is used to refer to someone from the Iberian Peninsula, while latino can refer to anyone from a country that speaks a Latin-derived language — and sometimes specifically to people from the Lazio region of Italy. In their totality, Spain and the Spanish-speaking countries of Latin America are every bit the melting pot of races and ethnicities that the United States is. The societies of Spanish-speaking Latin America descend not only from Spaniards and indigenous Amerindians but also from peoples of Africa, Asia, and non-Spanish Europe.
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Knowledgeable sources stated that the answer to this questions is a.manrique.  hope this !