By the time that Haven opened the 1st war, there was already problems with shipping going through the terminus to Trevor's Star---Haven had attacked and taken San Martin. Manticore had places a number of restriction on Haven ships using that access to the Junction and various customs restrictions on cargo. You don't have to raid their commerce to close off materials.
Once the war started, nobody went though to Trevor's Star. Anything that Haven was doing with systems in the SL it had to do it the long & expensive way- by hyperspace all the way. Not a particularly tempting target for SKM to send anti-Haven raiders to intercept shipping to or from Haven that way.
It is possible that SKM could have sent raiders (probably Light to Heavy Cruiser size) into the outer fringes of Haven's empire of captured systems but Manticore saw more need to keep those ships for use in direct raids into Haven to take territory or defeat parts of Haven's navy.
Recall that was essential what Haven did when it sent those ships out to Silesia to raid - more than Manticorian shipping or they would have stopped looking like "just" bolder pirates- and have Manticore pressed by the the Silesian Confederate and lots of the systems and shipping companies to send more warships to do anti-pirate work....that is how the Honor's squadron of Q-ships got built and dispatched to Silesia. Anything smaller that a Light Cruiser is not going to have the room to carry people to be put on prizes as crews to take them home and Manticore wasn't likely to just have it's anti-commerce raiders just destroy the primarily merchant shipping along with it's crews.
