The Republic-class is in the mod, it was just never used. If you want to use it, you can search for it in NewRepublicSpaceUnits.XML (?) to activate it, or try my submod, which has the Republic-class as an Era 3-exclusive buildable.
There are several good reasons why it is not used in the mod. It has very similar armament compared to the Nebula-class, and it is truly the king ugly of star destroyers. Then again, since the upcoming 2.2 will have the equally hideous Secutor and Gladiator classes, maybe there is some hope for the Republic
. Warships aren't supposed to look beautiful anyway.
Interestingly, despite some similarities to the Nebula, the Republic-class fulfills a distinctly different role in the fleet. With twice the number of ground troops carried, consumables for two years and being one of the least expensive star destroyers on the market, I've always viewed it as the NR's attempt to rapidly expand its fleet by directly replacing the Imperial-class. Yet while it is different enough from the Nebula, the problem is that neither ground complements or consumables carried are used in EaW, so all that remains is a cheap, disposable glass cannon star destroyer.