A few reasons:
1. The press has indoctrinated itself to despise stereotypes, partly for ideological reasons, partly for economic ones -- "Man Bites Dog" is a better headline than "Dog Bites Man." Yet another Palestinian Muslim terrorist is a "Dog Bites Man" story.
2. Another reason is that Major Hasan is such a classic example of "dual loyalties." We've all been told over and over again that the entire concept of dual loyalties is a baseless anti-Semitic smear and therefore doesn't exist.
In truth, of course, multiple loyalties are an unavoidable reality of life, which is precisely why George Washington's Farewell Address (the single most carefully considered utterance by the Founding Fathers -- it was worked on over four years by Washington, Hamilton, Madison, and Jay) devotes so much effort to warning about how to handle them. For example:
Against the insidious wiles of foreign influence (I conjure you to believe me, fellow-citizens) the jealousy of a free people ought to be constantly awake, since history and experience prove that foreign influence is one of the most baneful foes of republican government. But that jealousy to be useful must be impartial; else it becomes the instrument of the very influence to be avoided, instead of a defense against it. Excessive partiality for one foreign nation and excessive dislike of another cause those whom they actuate to see danger only on one side, and serve to veil and even second the arts of influence on the other. Real patriots who may resist the intrigues of the favorite are liable to become suspected and odious, while its tools and dupes usurp the applause and confidence of the people, to surrender their interests.
3. There's the Barack Hussein Obama angle. The press has a strong feeling that they must protect Obama from anything that could tangentially tarnish him. Of course, if they would just read Obama's memoir carefully, they would see that Islam never had any appeal for him -- it's too universalist. The President liked all of The Autobiography of Malcolm X until the uninspiring conclusion when Malcolm converts from the Nation of Islam to orthodox Islam after seeing the races mix on pilgrimage in Mecca.