Maybe I'm not understanding your question correctly. Do you mean "Why do people choose to learn the spoken English of America as opposed to the spoken English of other countries?" If that's the case, it's because America produces more movies and books and online content than any other country due to its 300 million plus population. People are most often exposed to spoken American English and therefore find it to be more useful to learn.

Try to express yourself better. What did you mean? Why do most people learn to speak with an american accent? why is that a problem for you?