The simple explanation is that then you would be assigning equal consumer interest to every character. The reality is the more well-known characters have historically sold in higher volumes. This seems to be becoming harder and harder to predict as the collecting community becomes more splintered and interests become more niche and as most of us already own a box full of Vaders.

I know it’s not so good for those who want to order a full case, but it is done to be able to create a flow, a gradual roll out at retail and create the ability to ratio a particular figure to 1, 2, 3 or 4 releases. This way they can [theoretically] offer more opportunities to find a higher demand character and not flood the pegs with the less in-demand figures. Of course what collectors want is those niche figures that will be issued in lower counts while the higher count releases will tend to be the Lukes and Vaders and Anakins.