Because it threatens diversity. Smaller cultures are changed forever or even disappear altogether - this is a real shame. I'm into linguistics, and now I know that there are about 3000-8000 languages on this world, of which thousands are on the "endangered languages list". Come on, that's a shame.
All those little things that make this big and diverse world interesting are disappearing. When I go to let's say south africa, I DON'T want to eat at McDonalds... I want something typical for that region.