I checked my files and note that Lockheed had the Starlifter tested and certificated to meet the FAA's standards for civil airline operations. While I do not think that they had passenger operation in mind in the short term, the fact that it offered easier loading and unloading than its 707 and DC-8 rivals meant that Lockheed could offer the Starlifter to civil cargo operators as a serious alternative to Boeing and Douglas.
Incidentally a civil version was ordered by Slick Airways, but they were absorbed by Airlift International and the order was cancelled, probably because the 707's and DC-8's offered a far superior payload, not to mention cheaper operating costs.
The civil-registered Starlifter was transferred to NASA, where it served as a flying laboratory.
So, by deduction, it would appear that the Starlifter was expensive to operate, while its competitors could lift a 50% greater payload. In service, however, it was discovered that the Starlifter's hold was full before the weight limit had been reached and this led to a fuselage stretch with plugs inserted fore and aft of the wing and thus the C-141B was born in 1977. All pre-'77 aircraft were then up-graded to 'B' specifications.
By then the Boeing 747 had been in airline operation for 7 years and Lockeed had long since introduced the Tristar and therein lies your answer. There was simply no need for it.
Behind every "no" is a "yes"