The only obvious answer is that something constrains ships to this plane; since this is a universe that has "Ether" and several new types of energy found only in the depths of space I'm tempted to visualise it in terms of convex "discs" of destructive energy above and below the plane of the ecliptic, at their thickest and most dangerous in the inner system, gradually getting thinner and less obstructive as you get further out, until it's safe to head "up"
Any suggestions / alternatives?
OK, I'm an idiot - blame lack of sleep. I mixed up Polaris and Sirius! I just need a reason for ships to be much slower from Earth to Neptune than from Neptune on out, and that's easily justified via Ether currents or something.