To elaborate on Human's point more, it allows for a more stable metagame... I.E. if Inconsistent Glalie is released during the third stop of the VGC, the metagame will be far different from that of the two preceding it.
Plus do you really want to deal with a huuuuuge number of pokemon? or would you rather have only 75 viable pokemon to worry about?
Plus do you really want to deal with a huuuuuge number of pokemon? or would you rather have only 75 viable pokemon to worry about?