Here is my interpretation of this weird decision. I may be right or wrong. We will never know.
The RSF got nervous when they saw Zagitova struggling this season, particularly the last two competitions where there was no Japanese challenger. However, they didn’t want to leave off the team the reigning OGM and GPF silver medalist, mainly because it will look bad (though they had no problem leaving off the 2016 team the reigning world champion Liza).
Remember that last year when Med had to miss the nationals due to injury, the RSF immediately started promoting Zagitova and she started getting high marks. It hurt Med in the end but the RSF got their OGM.
They do believe in the mental toughness of Medvedeva, in spite of her poor showing in almost all her competitions this season. They believe in her more than they do in Zagitova.
They don’t have that level of confidence in Tuk. She has had an up and down career. She was solid this season, but then pneumonia struck and all their doubts rushed back in. She skated well enough at the Russia cup final to prove that she was back, but it was too late because the RSF had already made the decision to send Med to worlds. The competition was a farce. It did not matter that Liza landed a clean triple axel, the only weapon that is strong enough to compete against the Japanese ladies. I suspect it was too late when she went down with pneumonia and had to miss the nationals. The RSF political machine had already started the process to replace her, and Med was the only strong candidate for that. I see the parallels of Med-Zag in the Olympic season.
Medvedeva’s marks at the Cup were too high. I believe that the RSF politicking machine will put their capital in Medvedeva rather than in Zagitova. Samudarova is a forgotten entity at this point. She could skate well at worlds but will be held down to make room for the Olympic silver medalist.
It is possible that Med will skate well enough to earn a medal at worlds and all will be forgiven, but the way this whole thing was handled, disgusts me. At this point I really hate the RSF, but then it has happened before and it will happen again. Fairness is never important to them. Only medals are.
Federation's Lakernik was called an "old-fart" (grand-pa) by fans and it got in the press, he is mad and offended, and told the next journalist that he will not talk at all, or explain anything about the selection process, until "they" apologize..... 
, how could he not comment..
