Coplanar orbits are very common in astronomy. From the small scale (moon orbits) to the large scale (galaxy discs), and this is due to the conservation of angular momentum. If you have a cloud that is being exerted upon by force (gravity), the conservation of angular momentum will cause an initial spin to be amplified. As the cloud spin speeds up, it flattens out into a disc, which is the maximal balance between gravitational collapse and centrifugal force.
In the sense of our solar system, the planets were formed through the aggregation of particles spinning in a big disc, these particles coagulating together to form the planets we know. That explains why they all follow essentially the same orbit plane (with the exception of the planetoid, Pluto).
When you look at the bigger scale, say a galaxy, you'll see this coplanar orbit mechanism persist through disc-shaped galaxies, such as the Milky Way, or Andromeda... even the spinning accretion disc of a black hole.