Why do planets follow the ecliptic?

The ecliptic is the plane on or near which lie the orbital paths of the planets in our solar system. (The reference link adds that Pluto is an exception, but Pluto is now not considered a planet.) This suggests that the formation process of the solar system resulted in a disk of material out of which formed the sun and the planets.

http://hyperphysics.phy-astr.gsu.edu/hbase/eclip.html
https://en.wikipedia.org/wiki/Formation_and_evolution_of_the_Solar_System

Our solar system originated in a nebular cloud which rotated and contracted until the center formed the sun and the remainder coalesced into planetoids and later the larger planets we see today.

All the planets rotate counter-clockwise around the Sun and the Sun and all the planets except Venus rotate counter-clockwise around their axes.

We see this pattern in our solar system as a whole: the orbits of the planets are aligned with the solar equator and we give it a name - the ecliptic plane!

By the way, the ecliptic of the Milky Way galaxy is tilted 60 degrees to the plane of the ecliptic of our solar system.

http://www.aerospaceweb.org/question/astronomy/q0247.shtml
http://www.earthsky.org/faq/where-is-the-ecliptic-in-relation-to-the-milky-way