Running a program with fixed-rate 60hz (a lot of games) would cause judder. It's the same sort of reason they don't do 150% scaling; you want integer multiples

Running a program with fixed-rate 60hz (a lot of games) would cause judder. It's the same sort of reason they don't do 150% scaling; you want integer multiples of the "low-fi" version so that you can upscale without artifacting.