I'm pretty sure the rules were set up when WTT was created so that the made-for-TV competition wasn't blown off by the strongest competitors. The first was held in 2009 and -- please correct me if I'm wrong -- the number of post-season tours in Japan wasn't as high as it is now or as lucrative. The ISU hadn't cut the prize money earned in GP's and possibly championships yet, and I don't remember the number of SOI dates being as low; 2009 was right after the merger with Champions on Ice.
Also, like the first Olympics Team Event, I'm sure when WTT started that the skaters needed to be convinced they would have a blast, and it would be worth the interruption. Skaters had their post-Worlds routines set -- vacations, getting new choreography, getting their heads out of their current choreography, healing injuries, getting ready for tours -- and there was no guarantee WTT would be nothing but a good deed gone punished.
It's who the crowds will enjoy, and performing at Worlds is in most fans' minds. Kvitelashvilli was invited to Japanese tours, so his inclusion will be a treat.