Because the emperor says he wants Luke to strike him down (his journey towards the dark side will be compleet). It would be majorly stupid to try and hit him then, wouldn't it? He wants a confrontation with vader as in empire.

EDIT: That's how I see it... But I don't think Luke strikes to try and kill vader. Besides he can't redeem vader by killing the emperor, it would probably have the opposite effect. And there's no need for vader to protect the emperor. I think the emperor could easily deal with luke himself. Blasting him away with lightening or something...