Why does the first film Clearsky stray from the orginal plot in the game?

In the Stalker "Clear Sky" game the player takes on role of Scar, who is searching for and attempting to kill the main character in the game prequal. In the Stalker Film Series I want to follow the Strelok's character plot line thoughout the entire series to give the audience a sense of connection with the character and the story. Switching between mulipule main characters between each story breaks up the audiences connection with the main character's story arch.