The rebellion of Korach is not a small offense but a great personal failing like the sin of the Golden Calf. It is a mortal blow to the continuity of the Jewish people and to its very survival.
Ordinary mortals have ordinary failings and self-interest is one of those failings. Moshe is true and his Torah is true. After that, no matter how fetching the slogan or how glorious the promise, caution and wariness about the person and cause being advocated are the proper attitudes to embrace.