Why is vosotros not commonly used in Spanish?
It used only in Spain, and if you're learning a Latin American dialect,your teach will likely only mention in passing.
"Vosotros" was not originally used in the south of Spain, and since most ships to the Americas departed from Sevilla (south of Spain) for over 200 years, the accent and other features of the language in Latin America are closer to the language of this city than to the rest of Spain. While "vosotros" spread to the rest of Spain, you still meet people in the south (at least in Seville) who avoid using "vosotros", like in Latin America.
"Vosotros" is not used because it is not useful outside of Spain. However, most conjugation tables include it, probably because it makes the whole personal pronoun structure of the language very symmetrical.
It´s used more than Uds. in Spain. When I lived there, vosotros is what you would hear 99% of the time in the street.
hmm this is interesting because i have a friend from guatemala and he says that they use vosotros there-- in fact, i was watching him send a text the other day and noticed him using vos
You're right vos isn't vosotros and they use it in Colombia also, at least in Medallín they do.