>>32042667Because they look great together. They're both beautiful as fuck, and they get a shit ton of development together, by Pokemon standards.
May/Brendan is nice, but it's not as good because Brendan too eccentric, while May is too cute. They also don't have many moments together. The few times they meet, May just gives you an item or battles you, tells you what to do next and disappears. During the more important moments, Steven takes her place. Not to say Brendan/May don't have any moments or development together, but not nearly as much as Calem/Serena.