Bear spray us easier to hit with, because of the sensitivity of bears' nose.
Bear spray is also easier to direct into the target. If you shoot a handgun and see no effect, you really don't know where the bullet went. With the bear spray, you can see the stream and move it into the face of the attacker regardless of how many legs it has.
I don't know about everyone else here, but I don't trust myself to be able to shoot like the Lone Ranger if a bear was bounding at me.