On the current patch, why is Xayah considered to be a good pick regardless of the support, while Rakan is considered to be only good while supporting Xayah? What is it about their kits that makes them particularly good as a duo while making Rakan dependent on Xayah?
Xayah has decent damage, CC and after level 6 guaranteed damage immunity making her a good ADC pick that can follow up on ganks with more CC, maintain damage and also have the occasional escape when needed.
Rakan on the other hand has abysmal range on his autos, needs to get right up to people to CC them and has a short ranged escape after CC that needs another champion nearby (Unless he's with Xayah, where he gets an absurd ranged escape).
So when laneing with Xayah, Rakan is able to pull off his CC, get his heal started and dash backwards to safety to get poke off, can zone enemies from farm while still having a safe escape available. This buff of Rakan/Xayah is increased as one of Xayah's abilities also procs off Rakan as well buffing the damage of the duo as a whole.
Keep in mind even though Rakan is a less than mediocre pick as of the current patch he is by no means a bad support, he just gets more utility with Xayah.