Because all American history is "white history."
George Wasington - White
John Locke - White
Benjamin Franklin - White
Thomas Jefferson - White...sort of
Every last president, prior to the current, - white
Every major contributor to the U.S. - White
Now get the average person to name twenty African Americans, Hispanics, Native Americans, Asians, Middle Easterns who contributed to the U.S. in some way? Most people can't.
There is more than just Black History Month - however that is just the most widely recognized (and criticized despite nobody is being forced to celebrate it and most don't...)
There is all Hispanic Women history month, Asian History Month, Women's History Month, Native American History month, Hispanic History month, the list is endless.
I don't understand why having x-ethnic History Month is so bad. God forbid you might have to learn something new.