The Nebula was designed as a heavy destroyer/capital ship.

There are certainly a couple differences that should not be overlooked. The argument of the ship specs is certainly a valid argument, as the mod already has limited build room, so any overlapping unit roles should be scrutinized heavily.

It's certainly been a while since then, so perhaps EvilleJedi has skinned it, or someone else has a good model/skin they'd be alright with donating.

EvilleJedi had a model for the Nebula, but hadn't skinned it yet.

The only visible differences are the turreted concussion missile launchers on the Nebula, the heavy turbolasers being duals on the the Nebula, two less tractor beam projectors on the Nebula, and it being able to hold 60 'Starfighters/shuttles' compared to the Republic's 36.

The armaments are almost exactly the same.

It's too similar to the Republic-class Star Destroyer.

After doing a quick forum search, the conclusions I came to as to why it's not added are: So I was just reminded of the Nebula-class Star Destroyer and its absence from the mod.

Defender class star destroyer