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.

https://en.m.wikipedia.org/wiki/Protoplanetary_disk

https://www.vanderbilt.edu/AnS/physics/astrocourses/ast201/o...