The ship's lore. They're not cheap to make and they're not manufactured in large quantities. Since heavily-armed luxury vessels serve little purpose for a House that's in the middle of the biggest threat to its sovereignty that it's ever faced, constructing ships like that is an extremely low priority.
The Renzu liner isn't in low circulation because the manufacturer isn't able to produce more of them; it's simply because the ship is so damn expensive that people can't afford to buy them.