I'm certainly no expert and I'm sure there's someone somewhere that can provide valid reasoning to support that. But these door skins, much like most of the other body panels, have a noticeable crown in the vertical direction. Sand enough in the horizontal direction and you will be adding flat spots, despite the overlap you may use. The other point to consider is the angle of your cross hatch. Here is a sample of a 45....
In this example let's go on the assumption that the spacing is approx. 1" apart. Now if we use a 30* angle on the cross hatch, and maintain the same one inch spacing in the horizontal direction, it gives us a closer overlap in the vertical direction for a more effective blocking session to remove those "flat spots" being added by a flat sanding board.
Where this has a better overlap and does a better job of getting rid of those flat spot issues, you can still feel the ridges in the primer after a session of horizontal only blocking. By performing a few vertical blockings, it cuts those ridges down, but also now look at the overlap... which should give the best results in blocking the panel.
Perhaps this method isn't suitable for all panels, but for the door removed from the car, it did well in removing the ridges..