Because the United States is as diverse a country as it is.
And the First Amendment establishes the separation of church and state, we the people petition our government to remove the words "Under God" from the United States Pledge of Allegiance. We cannot expect that a nation which harbors so many different religions (and many non-religious people) to endorse a particular group of religions by naming their God in the nation's pledge.
So, it is in the name of the very Constitution that this country was founded on that we the people call for the words "Under God" to be stricken from the United States Pledge of Allegiance.