(a)	Because Latin is the official language of the Church. It was adopted as such because it was at one time the official language of a large part of the civilized world.
(b)	Latin is a dead language and therefore the meaning of words remains constant.
(c)	To preserve the uniformity of the Mass in different countries and at different times. Latin makes it possible for a person to be able to follow the Mass as easily in China as in America.