Cos it looks battered and grubby. And would be the first time we see a ship built in an asteroid, as opposed to stuck in one or hidden in one.
To paraphrase Data...
It is...Brown.
But that could just be it's hull colouring, and the asteroid starbase is just something from that Ralph MacQuarrie concept art.
It was never meant to be anything other than a cool Federation outpost, originally. All this stuff about a secret base is jumping the gun a bit. We already know that the Federation builds outposts on asteroids - for example the ones destroyed along the Romulan border in "Balance of Terror" - I imagine they have a ton of asteroid facilities like this, as they are convienient.