I think they don' build alot of them.
The A.I. is programmed to construct some ships more than others. The Cube, being the largest ship in the game, will probably be last on it's "to do" list.
You can modify this, but I'm not sure specifically how, by altering your game files.
Fly about, whack some Feds, then come back. There might be one built by then.
Azrael