>>49833516They have great designs and they're made more immediately interesting by being weirdos but the bulk of the lasting appeal I believe does lie in being twins.
People care about characters who care about each other. A large chunk of internet fandom culture is about relationship dynamics more than individual exploration of character. People will gladly make up pairings of characters who barely know each other, but when they actually do know each other in canon that's a big bonus.
With writing as anemic as Pokemon, you rarely get characters where it's believable that they love and care for each other. Personally, this prevents me from getting invested in most Pokemon humans beyond surface-level design appreciation. But with twins, the writers get almost a free pass. Of course they're important to each other. It's so easy to imagine different relationship dynamics. And because this is still Pokemon, and the writing is still anemic, you're free to project whatever kind of love you want to see, be it wholesome family stuff or fucking degeneracy.
