Because maybe that period there was a lot of European designers on the team. The aircraft was also in service with the French and The British.
Its not really a headache what ever the gauges are calibrated in. You just keep it in the green band and do something when its in the red.
The only time F is better than C for a pilot is with dew points. C is to big a unit apart from that you could have all the gauges in bunches of bananas per donut squared or even a none dimensional % of the max and it would do for operating. But the engineers like numbers which can relate to thermodynamic output so C or F it is.
To be honest the yank engineering has always been a bit of a mixture of imperial and decimal more decimal these days. Some industry's are more likely to use it than others depending what their export market is. The only one in my experience which is 100% imperial in the main is the oil industry.
But all US international company's are struggling remaining on imperial. Nobody else has machine shops etc setup or calibrate to manufacture in imperial. Most engineers from the 1970's onwards in the rest of the world don't have a clue about imperial measurements or working with them. The 787 had issues with it which caused delays with it coming in service due to this. So if they intend to make use of foreign production facility's they virtually have to work in metric.
One of the lads off my degree course went on to work in NASA. And when being asked about getting his brain screwed with converting everything said that they worked in metric all the time. And the only time stuff got converted was for outside NASA reports for US consumption. I presume he was correct as they had quite a famous screw up and managed to crash a satellite into a planet due to one team working in imperial and another in metric.
Now I am a metric trained and educated person so will admit I am a bit bias. I can't see the rest of the world changing back to imperial. Personally I agree the C unit is to large for aviation and F works better for one situation. But the rest of the time it doesn't matter. I have flown both F and C calibrated aircraft.