I would wager this is a display scene issue over a scale issue. Plus the pricing issues...
The barge, Unicron, the Razorcrest, the Sentinel, Galactus, and the Sky Striker all lend themselves to big display scenes with a lot of dynamic visuals. I will always love rancors in star wars, and even though they exist outside of Jabba's palace the story of a rancor is limited to the interaction with Oola, Luke, a Gammorrean and Mallakilli. I don't really have a desire to display a rancor with tons of characters around it the way I might with the Barge, Unicron, Galactus or a Sentinel. Those items all feel like a centerpiece to me in the way a rancor does not.
Plus I have had a rancor since 1984. I love my old school rancor, but the beast itself as a concept isn't a grail item in the way these other haslab projects are.