I don't know why,you are going to have to ask W why. The constitution does read, in case of extraordinary occasions he can convene both, or either, houses. Maybe he doesn't feel circumstances warrant calling a special session. It isn't something to be done lightly. Below is the section dealing with convening congress under special sessions.
Section 3. He shall from time to time give to the Congress information of the state of the union, and recommend to their consideration such measures as he shall judge necessary and expedient; he may, on extraordinary occasions, convene both Houses, or either of them, and in case of disagreement between them, with respect to the time of adjournment, he may adjourn them to such time as he shall think proper;
Looks pretty clear to me, I don’t get it. He must have some strategy as you suggest.