Long story short - because it's deeply un-American.

Atheism is fundamentally at odds with the values and ideals of the founding fathers and the vision they had for America.

Remember this is a country that was born because "all men are created equal, that they are endowed by their Creator with certain unalienable Rights".

The idea that all are created in God's image therefore all are equal - is a foundation for why our country exists in the first place. That's a fundamental gene in the American DNA.

Moreover the founding fathers were either A-devout Christians themselves or B-held considerable regard for the Christian religion.

Yes there's the odd quote about the shortcomings of organized religion, Jefferson's Bible, Ben Franklin's religious aloofness - however, the founding fathers were overwhelmingly what most today would consider 'devout' Christians.

Take for example, the members of Constitutional Convention of 1787 (which authored the Constitution) - 96% were Protestant, %4 were Roman Catholic. A fair portion of these were actually trained for ministry.

Or this quote (among many others) of George Washington - "I am sure that never was a people, who had more reason to acknowledge a Divine interposition in their affairs, than those of the United States; and I should be pained to believe that they have forgotten that agency, which was so often manifested during our Revolution, or that they failed to consider the omnipotence of that God who is alone able to protect them." ...in which he literally outlines his abhorrence to the omission of God from the American consciousness...because of what God has done for it.

This is not to say atheism is 'wrong' or religion is 'right' or vice versa.

It's just to say that American and Christianity have been historically intertwined.

It should be self evident why atheism is considered a taboo here.