How would the pilot interpret the frequency change if it was expressed as a channel number? If new radio gear had to be fitted which shows channel numbers the cost would be frightful. The alternative would be that he had to look it up on a table to determine the frequency then input it in frequency format to the radio. Where's the benefit?
I have used channel numbers to great effect in the past, but they were employed by base aircraft with radios which only had half a dozen channels on the selector. When they left base area frequencies had to be given in standard format for them to use other equipment.