I think because they are basically just re-skins with a few BOFF and Console changes and also that they aren't the top rewards for gambling with the lock boxes. Personally I prefer the Mirror ships to the Tholian ships, Tholian ships have no place in Starfleet or the KDF, goes against the entire feel of the Tholian species established in the series and books.
The 18% - Own it! - Be proud of it! - Make it grow!
Players who do the Defera Hards, if a team helps you complete a Temple run say thanks, it's the decent thing to do as they help you out of the kindness of their heart.