For me the answer is pretty simple:
The Lucasarts adventures were among the first games to successfull feature emotionally driven game dynamics.
I liked some of what Sierra had to offer, but the design of the Lucasarts adventures were far more elegant (you never died, so the narrative always felt seamless), and featured solid, effective writing.
Broad humor like slapstick is often one of the easiest things to convey in a new medium (in everything from early movies, to mobile games), and the Lucasarts game did it very, very, well. Games like Loom, and The Dig started to explore deeper emotions, and proved that games could tell a powerful story.