Because the pressure line is on the top and the bleed screw is on the side. If you're able to make an air bubble float out the side of the slave cylinder, then you're much more talented than I.
Maybe it would help to have the car pointed down a steep hill.
Even more of a problem is the clutch master cylinder, which is mounted vertically next to the pedal. The supply line is on the top, and the pressure line is near the bottom. I can't see any way that pedal pushing could get all the air out.
Both problems, however, are easily solved by reverse bleeding.