This is beyond ignorant.
SKT T1 2014, FNATIC, and TPA only got 5 skins because at the time these skin lines were made, they ONLY made skins for the main players who played in the Worlds.
However, starting in 2015, Riot announced that BECAUSE there were usually 6 members of the team since a substitute player was allowed on the roster, they would do a skin for the substitute member as well, since they also played in worlds and should get recognition for stepping in when one of the usuals had to step out for whatever reason. So that is why starting in 2015, 6 skins were made instead of the previous 5. They also started making ward skins to represent the coaches for the team in 2016.
So when SSG won, their ADC and Substitute selected Xayah and Ezreal, while their support selected Rakan. Xayah wasn't simply added because Rakan was selected. One of the other members of SSG selected her - and if memory serves, he did so because the support selected Rakan.
So then we get to IG in 2018. The top and substitute selected Irelia and Fiora because they wanted to. The substitute team member didn't select Xayah, probably because he didn't play Xayah, or simply probably because he really wanted a Fiora or Irelia skin if they were one of his favorite champions. There was nothing unfair about them selecting the champions they wanted, so your baseless claim that they should have chosen Xayah is not grounded in any sort of reality, nor are you allowing the members of the team to pick who they want as their own reward for winning the biggest tournament in League's yearly event schedule.