In order to witness a transit, the orbital plane of the planets must be at or near our line of vision, but not all solar systems have the same orientation. (discovermagazine.com)
But the real kicker for the researchers was the fact that their simulations also predicted that there would be objects in the Kuiper Belt on orbits inclined perpendicularly to the plane of the planets. (sciencedaily.com)
We see this disk as a dim glowing band, called the zodiacal light, running along the plane of the planets. (discovermagazine.com)