If you really want a lore reason, it could just be said that the ancient ringworld builders made automatic solar system wide protection systems throughout the universe. They are so advanced that it is treated more like a universal law than technology. An attacker simply have to stay in the target solar system for a full week, reading an ancient war declaration poem that works to disable the protection system upon full completion. If the poem fails to be read completely nothing happens and the defender remains safe.
In other words, I am pretty sure we can think of some lore for whatever idea we come up with. The mechanics are what matter.
In other words, I am pretty sure we can think of some lore for whatever idea we come up with. The mechanics are what matter.