Because the writers didn't want him to >:
Anyway, at the time his priority was the student council and at the time what would he have made her do? Their plans were not that far advanced yet so Lelouch could randomly geass her or he could not and save it for later when he would have been in deeper trouble and could use her better.
The problem with anime it has has to fit in a certain script, in the manga if I remember correctly she was geassed into doing something destructive.. I don't fully remember what it was either poison gas or something with missiles
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