You didn't have to be more popular; even next most popular was a death sentence.
He was also suspicious of anyone from Leningrad, it was too big, and too far away from Moscow. He had many of the leaders of the defense of Leningrad executed after the war.
IIRC, Stalin had potential rivals edited out of photographs to deny that they even existed. So not only would they kill you, they would edit you out of photographs and kill the person who doctored the photos if the edits didn’t look convincing enough.