I have posted on this topic in the past, some thoughtful, some humorous. But for me the reason is pragmatic. Besides ATA, I belong to and shoot at a number of clubs. Some of these have down range curtains to prevent shot from going into wetlands, or off of their property.
And in one case the club has limited shot size to eight or smaller pellet size. This is because the combination of distance, height of curtain, strength of curtain material and prevailing winds requires lighter shot. Some 7.5 loads will punch holes in the curtains, and they are expensive to buy and even more expensive to install.
So given the need to buy various speeds for various uses, and using several thousand rounds a year, i just buy eights to keep the inventory permutations down.