It happens because GP hosts are the ones who do the inviting. And because the only way to be guaranteed two non-host spots is to place top 10 at Senior Worlds. So athletes like Eunsoo, who are 1. from a non-host country and 2. only just now age-eligible to compete in seniors can be easily skipped over if countries wish to skip them. And countries with athletes that are in the running for the Final in a specific discipline have every reason to skip top competitive athletes if they can. It happens with athletes from host countries as well when a field is too deep and there is more than one home athlete deserving of the host spot. McNamara & Carpenter didn't get a second berth last season although two went to Lauriault & Le Gac, Abachkina & Thauron, and Kaliszek & Spodyriev--all teams with lower SB scores. Stepanova & Bukin didn't get two after winning Junior Worlds either. It also happens to high quality athletes coming off injury, like Coomes & Buckland, who miss Worlds during a previous season. Sometimes previously injured senior athletes can mediate the issue because they are famous enough to sell tickets; but not everyone can. And even if they can, it may not be enough incentive for a host country to risk having his/her own star athletes bumped out of the Final. Host countries can swap spots for their own athletes so that's often an advantage for some athletes. However, many of the host countries also have deeper fields, a swap may not be viewed as worth it by another country if the athlete in question is competitive, and junior athletes have less base value in order to earn truly comparable season's best scores. They often come in ranked lower than they should be, and everyone knows that some of these athletes can really score higher.
Why do host countries have the power to decide many of their invitations? Because they foot the bill.
