I always thought,
The Leviathen was a republic ship. Once Saul Karath diffected to Reven/Malak they used the Star Forge to 'mass produce' it. I imagine the Sith quickly hit the Republic ship yards so they were unable to build more [Leviathan class]. Also without the resources of the outer worlds it seems likley that the Republic had to rationalise and produce 'lesser' vessels, ie. the 'Endor Spire' class.
Of course we have some contradiction here,
KOTOR tells us that the Leviathan was a 'Republic' ship, however it also tells us that it is of 'Alien design'. Maybe the Star Forge was used to make some improvments? Or maybe it was 'Aliens' ie. non humans, within the Republic that designed it. The Republic comprised numerous non human groups, so maybe it was made by them for Republic service?
The Star Wars Databank is quite clear in stating that the Leviathan was a Republic vessel.
Of course its all just speculation.