I think it’s because Sierra at that time was like one big, happy family! If you look at Sierra games from the late eigthies and early nineties you’ll often find them referencing each other. Even Police Quest had a comic moment where Sonny Bonds (the hero of Jim Walls’s PQs) puts on a white suit and some text pops up referring to him as Leisure Suit Sonny. And of course Space Quest had you crashing into Daventry Castle’s moat if you pushed the wrong button in the escape pod. Not to mention the Sierra factory, filled with Sierra characters, Larry enters when he dies in LSL1. Great gaming moments, all of them!