Sometimes it may not be visible on the surface, but there's a lot more to getting a ship to game-ready status than just having it look pretty from a specific angle and lighting on a screenshot. The ships that were ready were released and the others, well.. that's the whole point of episodic releases.
The mark 2, for example, is an iconic ship to the show. It doesn't have a cockpit yet, I'll build it myself when time allows; but also, releasing the show's iconic fighter only to see it in multi would be kind of lame. So we decided to hold that one back until we have it 100% completed and can put it in a story where it makes sense to fly it without it feeling forced in just for the sake of it (which would have been the case if we used it on the Theseus in R1).
The Pegasus isn't much more than a placeholder. The model is old by today's standards (BtRL age, and it was a placeholder back then), and even if you found it nice in the old screenshots do remember you didn't really see it up close. It looks downright horrible from close range, and that's the least of it's problems so it's quite possible it'll get rebuilt from scratch so it's quality can match or surpass that of the Theseus.
The Colonial One I have to re-check, but it's possible we'll put it in R2 - but don't hold me to that just yet