When you are set on something, sometimes it is hard to find out you can't have it and ignorance is bliss in that situation.
I am guessing he fears what they might say, and wants to live with the thought that he CAN make it back, until it is proved otherwise (in Spring Training).
Also since the original thought was he could try in Spring Training before having to make the decision based on the insurance deadline, he probably wanted to give him time to re-hab more to give doctors a clearer picture, and figured it was too early to tell for sure.
Now that the truth is out (about the deadline) a certain amount of panic response is going on, on both sides.
That is my guess. I do like that Bagwell's agent recongizes why there might be some push by the Astros for a definative answer before the insurance deadline. It was also good to note that there is not as much animosity between the two parties as might have been thought. Both are stressing a bit over the situation for different reasons though (each over their worst case scenarios).