I believe it is the fact that the solar system was once a spinning ball of gas and dust, and the spinning motion caused the cloud to flatten out into the protoplanetary disk. Like flattening a ball of pizza dough by spinning it in the air.
When the solar system formed, the gas cloud collapsed along the shortest axis first into a spinning disc and then formed the planets and sun. This naturally aligns the largest bodies in most systems along the astral axis.