Spelljammer: Adventures in Space, specifically the Astral Adventurer Guide pp. 26-57, adds more ships. All of these ships have damage threshold 15, except for the Bombard and Tyrant Ship with damage threshold 20.
Notably, while the AC of the ships is determined by their primary hull material, this does not seem to have a strong correlation with damage threshold. The highest-AC material listed for a ship is metal; four ships are of metal construction, but the Bombard and Tyrant Ship are not among them. The hit points of the Bombard and Tyrant Ship are also only middling.
The best argument I can think of for the higher damage threshold of these two ships is that they are likely to be constructed with especially thick bulkheads: the Bombard, to endure the stresses of firing its spinal cannon; and the Tyrant Ship being a carved monolith. However, I was not able to find any explicit justification in the text. Furthermore, a similar argument could be applied to the Turtle Ship which only has damage threshold 15, despite it being "encased in a protective metal shell" and being able to "travel underwater safely, even to great depths", with even the internal bulkheads and hatches apparently being strong enough to "prevent the ship from flooding completely".