Space

Likely something to do with aerodynamics and weight distribution. Aircraft and rocket design are very different, and the glaring problem with designing the booster to fly back like a plane is the fact that the payload is mounted on top, meaning you can't have a tapered end, since you've got to be able to mount a full second stage rocket on top with 20+ tons of mass. So the problem becomes, how can you make a functional airplane out of what is basically a giant steel cylinder with a flat top on one end, and rocket boosters on the other. Also when it flies back, it's going to be bottom heavy, as most of the fuel mass will be gone, leaving all the heavy rocket engine weight dragging it down.