The orbits of the planets are coplanar because during the Solar System's formation, the planets formed out of a disk of dust which surrounded the Sun. Because that disk of dust was a disk, all in a plane, all of the planets formed in a plane as well.
http://curious.astro.cornell.edu/about-us/57-our-solar-system/planets-and-dwarf-planets/orbits/242-why-do-all-the-planets-orbit-in-the-same-plane-intermediate
