Here's why:

* Angular Momentum: The cloud of gas and dust that forms a solar system has a small amount of initial rotation. As the cloud collapses due to gravity, this rotation is conserved and speeds up. This spinning motion forces the material to flatten out into a disk.

* Centrifugal Force: As the cloud spins faster, the centrifugal force (the force that pushes objects away from the center of rotation) balances out the inward pull of gravity, preventing the material from collapsing completely. This balance leads to the formation of a flat, rotating disk.

The disk is not perfectly flat, however. It has a central bulge where the star forms, and the disk itself can have some variations in thickness.