My take on an answer: they knew that they'd have to create a working rocket in just a handful of attempts (turned out that they'd hardly had money for the fourth - the first successful - attempt). An ablatively cooled nozzle was the easiest way to solve the nozzle-cooling problem and they deferred work on a more sustainable method to later.
Other pragmatic solutions were for example that the booster and upper stage have the same diameter so that the same tooling could be used. Or that the diameter of the stages is 12 feet because that's the maximum diameter that could be easily and cheaply transported by road to the launch site.
Other pragmatic solutions were for example that the booster and upper stage have the same diameter so that the same tooling could be used. Or that the diameter of the stages is 12 feet because that's the maximum diameter that could be easily and cheaply transported by road to the launch site.