I don't know why LucasArts wouldn't want want to do games based on the original trilogy, unless someone in marketing is telling them it wouldn't sell (which I wouldn't buy for a second).
I bought LSWII because LSW was such a fun game...and LSWII was even better (by a long shot). I think that LucasArts could definitely make some great games from the original trilogy if they wanted to - all they need to do is come up with some original missions to fill in some of the gaps. They could also involve some secondary characters to create other missions.
The possibilities are pretty much endless, but I think that LucasArts has decided to focus their attention elsewhere for reasons I don't understand.
Let every nation know, whether it wishes us well or ill, that we shall pay any price, bear any burden, meet any hardship, support any friend, oppose any foe, in order to assure the survival and the success of liberty.
~John F. Kennedy