That is one of the eternal questions! Why Not? Suppose He had ? But before that.. What if McDowell had moved his troops into Bull Run before Johnson had a chance to bring up his from the valley? Or what if Lincoln had chosen to Reinforce Ft. Sumter and make a stand there instead supplying the outer forts to downplay the tension?
Thats what makes history so facinating.. So people like us can ponder on it for the rest of our life..