If you look at an anatomy diagram, you'll find that the bony plane of the posterior superior iliac spine is larger than that of the anterior superior iliac spine. With a larger bony plane, the needle tip is more likely to land and "bed down" easily. With a smaller bony plane, the needle tip drops as if point to point, like needle tip to awn; it's easy to slip off and hard to keep the needle perpendicular.
Perpendicular needle entry is very important. A bone marrow puncture by definition has to go into the bone, and bone is hard, which means it takes effort. Perpendicular entry is like hammering a nail straight: the vertical force lets the needle go in more easily, so you can use the minimum strength and cause the child's bone the least possible damage.
