Four short months ago you probably claimed they'd use jetfuel-laden airplanes if they had them, didn't you?
And last week you probably claimed they'd use explosive-laden shoes if they had them.
Puh-lease.
One set is using a common capability in a unique fashion: the other is using a very UNCOMMON capability.
If I were an Evil Terrorist MastermindTM, I'd use the most destructive means available to me immediately; I might not be around long enough to use it otherwise. And a nuke might not be usable if I kept it lying around instead of using it right off the bat.
Incidentally, for more detail than you ever wanted to know about nuclear weapons, how they work, and the various logistical issues surrounding them, there is an excellent book by Chuck Hansen called U.S. Nuclear Weapons: The Secret History. The first chapter is a very readable explanation of nuclear physics and how nuclear weapons work. The second and third chapters are a history of the development of American nuclear weapons from 1945 to the end of atmospheric testing in 1962. The rest of the book discusses each model of nuclear weapon made between 1945 and 1987 (when the book was published), including some that never got built or were planned for production in the early 1990s. It should be available at any major library.