Because he's played by Bitch Macavoy? He's always played wimps. He's not enough of a good actor to play anyone different from himself.
For every lie I unlearn I learn something new - Ani Difranco
Well, I think that James McEvoy is a great Xavier. But to each his own.
Laugh while you can, Monkey Boy
As sensational as Patrick Stewart is, this is the first time a youthful Charles Xavier with various emotions and deeper layers is being showcased. James McAvoy portrays him as more than the stone stoic professor, constantly able to keep it together with silent, dignified air. Here, he's kind, generous, funny, flirty, impish, poignant yet pulls off dutiful, encouraging teacher all in one film. In fact, he seems most involved with his students and their growth here than in any of the other movies.
I personally also saw hints that he may have a touch of his mother's drinking addiction from the comics and I wonder if he craves someone looking after him the way he always has for Raven. He seems to look to Erik as a brother figure instantly, despite seeing the questionable acts that have been committed both by him and against him. Charles is seriously willing to leave all that in Erik's past and chart a new course with an infinitely more fulfilling mission.
Yes, he has his flaws as well, but the overall package is a character I love to pieces and will have trouble seeing Charles depicted otherwise in the future. Same for Fassbender's Erik.