Because Buzz did not discover the Moon or Mars. Columbus discovered a whole new part of the world no one even knew existed. It would be like if Buzz discovered that there were other planets in the solar system and that those planets are readily habitable and gave the directions to them. Even if Buzz did not discover the specific planet that would one day be the home of a famous colony, he still would get massive credit for pointing people in the right direction to begin with, a direction they never would've concieved on thier own
Then by that logic it should be called Leif Erikson Day. ---