Some possible reasons why Michael Myers hasn't talked again since the murder of his sister is that it's part of the catatonia he tricked everyone into thinking he had, while others, based on the events of Halloween Kills, suggest he is still in shock after what happened in 1963, and he's still that little boy who ...
Farrands expanded the "Curse of Thorn" plot line, in which Jamie Lloyd is kidnapped by a covert cult who has cursed Michael Myers via the Runic symbol of Thorn, which compels him to kill and also affords him immortality.