1. Timeline Compatibility: The X-Men First Class movie is set in the early 1960s, while the character of Jean Grey was introduced much later in the Marvel comics continuity, likely to avoid contradicting established plotlines and character history.
2. Core Characters Focus: The X-Men First Class movie focused primarily on introducing the younger versions of the original five X-Men, as well as Professor X and Magneto's backstories. Jean Grey's inclusion might have shifted the narrative's focus away from the core characters and their dynamics.
3. Origin Adaptation: In the X-Men First Class movie, the filmmakers decided to showcase different characters for the X-Men team compared to the comics. This creative decision allowed them to explore different relationships, powers, and story possibilities without directly referencing Jean Grey's complex psychic abilities and her future importance.
4. Timeline Consistency: Including Jean Grey in the X-Men First Class movie would have necessitated introducing her at a young age, which could have created potential conflicts with her later appearances in other X-Men movies. This decision helped to maintain the overall continuity and chronological order of the movie franchise.
5. Narrative Pacing: The X-Men First Class movie had a limited runtime and a specific story to tell, which included the emergence of mutants, the formation of the X-Men, and the rivalry between Professor X and Magneto. Adding Jean Grey's character and her backstory would have demanded more screen time and potentially disrupted the pacing of the narrative.