Crossing is easier because you can simply hold your finger in front of your eyes and look at that for practice.

Diverging requires you to look past the image, meaning you have nothing to really look at, which makes it difficult to figure out what your eyes are even supposed to do.

Those stereograms aren't helping much either, since they look like nothing until you get it right. With cross-eye you have instant double-vision that you just need to align.

Cross-eye also works across much larger distances, diverging fails when the images are too far apart.