1. Since they order starwarships from Imperium yards, it means that they don't have the industrial capacity to do it themselves.
2. Quality over quantity.
3. That they paramilitarize merchantmen could be either cultural or practical, maybe both, in that they can't get enough from the Imperium, or don't have enough money.
4. A subsidiary issue would be able to maintain technological level equipment, which if they can't manufacture it themselves, or not on a wide scale, they still can operate and repair.
5. Any uncovered plot to commit genocide is likely to trigger a counter genocide.