Obviously LucasArts games belong here. You know it, I know it and you can bet GoG knows it. I am convinced that there has already been contact between the two parties.
That LucasArts games aren't here yet, I guess, could be due to two reasons
1) Money. Maybe LucasArts doesn't consider it lucrative enough and they are only willing to allow GoG to sell their games for prices which GoG considers too high.
2) Rights. Maybe you need to do more than just get the permission of LucasArts to sell their games. Could be that the rights have been splintered and other parties are involved.
Whatever the case may be, I'm not worried. I'm sure they'll get here eventually, and there's plenty of stuff to play in the mean time.
Money I will buy as a plausable reason not to put their games on GOG but licensing is not as Lucas Arts owns the IP of every title they made save for Sam and Max which is owned by there creator Steve Purcell.