Why did Lockheed abandon the Constellation II?
Douglas probably already had a few commercial operators interested and close to signing orders when their tanker bid fell through. 

Lockheed didn't, and decided to not dump more money into what could very well become a black hole project that'd never pay off.

And it may well be that they decided the design wasn't feasible for commercial operators. Looking at the picture on Wikipedia, almost the entire rear third of the fuselage would have no place to fit doors or windows, making the aircraft useless for passenger operations.

And the air cargo market at the time was a lot smaller than it is today and still saturated with former military aircraft retired after the Korean war and WW2.