Because the Air Force doesn't want to give up its space assets to create an independent Space Force, much as the Army didn't want to give up its air assets to create an independent Air Force.
pffft! airforce... the spaceforce should be drawn from the submarine force of the US Navy.
I would think that the Airforce would be the natural father of a space force. The beginning space fighter/bombers will likely be like an oversized version of spaceshipone which could use a vertical rocket as it's first stage. It would have to launch from land, the Navy might have a claim when we build space-only battle cruisers, but that's not going to happen for a long time.