Originally Posted by Legion
Why? She's just a clone of Jean Grey...a pretty generic concept. If anything Mr. Sinister's motives would have made more sense in X-men the animated series if she was included.
Clones can make things very confusing, as seen with the Clone Saga in Spider-Man.
Plus Madelyne Pryor is part of the reason why the Summers Family Tree is so completely convoluted, Cable is the son of Cyclops and a Jean Grey clone, then the clone became an evil demon worshiper. Cable was then infected by techno-organic virus and was sent to the future to get a cure. Try selling that to the general audience.