Why did Apple kill off the 17" cMBP? We assume it was because it didn't sell in large enough quantities. What would be different with it being retina? Because of its expense it would still likely be a low selling niche product.
Game theory dictates that Apple should have kept the 17" MBP, so I don't know that there is a compelling reason. It must have been losing a lot of money for them to discontinue it.