Why is the solar system flat?

Sometimes we spend too much time focusing on what’s up close and we miss the big picture. Here’s a nice big thought for Monday Morning!

It’s no big news that the planets, moons, asteroids, etc. in the Solar System are arranged in a more-or-less flat, plate-like alignment in their orbits around the Sun. But why is that? In a three-dimensional universe, why should anything have a particular alignment at all? Why is the Solar System flat?