Partly a cultural thing. But also the pheromones etc. are present in fresh sweat. BO is caused by rotting sweat, basically. Poor hygeine in any animal is a sign of ill health and poor breeding potential, so this could be an evolutionary source for the dislike. But certainly previous generations hardly noticed. George the ivth is the first mmonarch to have complained about it...caroline of brunswick was a bit niffy. But Elizabeth famously only had one bath a year "wether we need it or no" so must have reeked. And you dont catch Spenser mentioning it