Because he wasn't strong enough and plus he was trying to block what was hitting him...luke at the time was rather inexperienced, but he still held his own really well against vader but I don't think Luke woulda won that battle unless he was as strong as he was in ROTJ
another perfect example of why the eu is a piece of shit.
Here is this guy clearing hallways with a cough, while luke, who is supposed to be some sort of force prodigy can barely move his lightsaber by grimacing at it for half an hour.
--Zero Punctuation's Review of the force unleashed.
Luke may have been able to balance a rock on top of Darth Vader's head... but it would have required a lot of concentration and it wouldn't have helped defeat Vader.
Well, can't exactly claim that Luke has had a lot of training at that point. Prodigy or not, training is still necessary. If you're comparing to Galen, Galen was trained since he was a child, unlike Luke.
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Its been a long time, how have you been? You know, when you MURDERED me.
Ahh..well let us put that behind us....for science....you monster....