So you don't think it's logical that the Colonia highway would be a supply and transit route between a) Colonia and the bubble and B) between the stations on the highway itself?
That's not the issue, considering the number of systems there are literally billions of possible routes to take between stations on the highway, any pirate activity would need to happen close to the stations to have any possibility of getting more than one ship passing through in a year, so having them between stations is not going to work, having them near the stations would make sense because pirates depend on passing traffic, and unless they can spot passing traffic from a couple of hundred LY away, which is not possible in ED, they aren't going to be doing any pirating.
Pick any random system between any two stations on the Colonia Highway and look on EDSM to see how many people have passed through it in the last year and that will give you an idea of the problem.
The essence is, pirates need to be near civilisation to actually pirate. It was easy on the oceans, due to currents, weather and land masses there were plenty of choke points where sailing ships and indeed modern ships can be pirated. Even today pirating only happens at those points, the choke points on the Colonia Highway are the stations, so that's where the pirates will hang around, not in the dark between the stations. That would be like putting a pirate sailing ship randomly somewhere in the middle of the Pacific Ocean and expecting other sailing ships to come sailing by to be pirated, not going to happen.