For the same reason that Barack Obama cannot be president, Ronald Reagan could not be president. It says so in the Constitution. See "The Constitution Say Obama Can't Be President. And Neither Could Reagan." Chris Kelly, Huffington Post, 2009-08-03
"No person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President..." U. S. Constitution, Article II, Section 1
If we are to believe strict constructionists, then the Constitution should be taken word for word without interpretation. The government declared the Constitution in effect on March 4, 1789. Thus no person born after March 1789 is eligible to be president.
The first ineligible president was the tenth president, John Tyler, born March 1790. The last eligible president was the twelfth president, Zachary Taylor, born November 1784. So, we haven't had a legitimate president since 1850!
Gosh, all the wars the United States has engaged in since and including the Civil War have been illegal, all the laws signed by a president after 1850 have been illegal, and all the Federal taxes raised since 1850 have been illegal.
Ah ha! Now I see the plot of the strict constructionists, they want to stop all taxes. Interesting that they don't want to stop all wars, wars that require taxes.
