@FiddleSix I'm genuinely glad you asked! It's most to do with the water temperature and the amount of agitation needed.
Cotton in general washes better when the water is hot and when there's a bunch of agitation, so a longer program. (On my machine the suggested program is 2h 20min at 60C, for example.) This is fine. Spin cycle it as harsh as you want to, too, as a plant fibre it soaks up a lot of liquid and dries slowly.
But when you wash synthetics, often containing elasthane and spandex etc, those do degrade very fast when the water is hot and there is a lot of agitation. Hot and long also leads to more microplastic shedding, I'm fairly sure! You may also want to use a different detergent for the two, or different amounts. They also dry faster, so less spinning required, less wear on the clothing and washing machine, and less energy expended.