The previous answer was most excellent. Full of wisdom, it was. :)
However, another answer is simply based on what I call 'the necessities of story-telling' - I don't know the regular term for this but here it is:
If Luke is informed as to his father being the great bad Lord Vader, then the plot as it is, and the revelation at a dramatic time, etc. is derailed, in any one of various ways (as the previous answer has already explained).
Also, quite simply, Luke might answer such an early revelation with:
If he's my Dad, then why don't we go seek help from him, since I'm his son?
Shut up Luke. You should really know better, by now.
Mess with plot, will you? consequences there are, author might kill you off.
[Oh deah, that would be so against all the proper protocols to be used when addressing a nobleman of the Empire. He would surely punish you horribly in some way, to emphasize his rank, and his right to be free of importunities from what he might term mere beggar-boys!]
or Luke might just say:
This must be a really bad man, to have neglected and abandoned me for so long. This confirms that we must oppose him completely and in every way.
[....and, the drama is made flatter by that much. No revelation, no conflicted emotions, no upsets, just at most, "What a bad 'Father' you were! take that!"]
In sum, revealing it too early would be unhelpful to the story.
And, the main plot elements of the ('early' i.e. Ep. IV-VI) Star Wars movies are somewhat dependent on reversals of the usual:
the hero's father is the main villain
the Princess is the hero's sister
the wise elder throws his life away for obscure reasons (-perhaps I'm wrong here. Sorry.)
the plot is all over the place - many places, many planets
and so forth. Having (1) and (2) known to all members of the plot early on, would take that much away.