Adam actually mentioned in the online Aftershow that one big reason they waited so long to do SW was being able to afford the clips.
And then he laughed. He was joking, I doubt the cost of those clips for the movies would have cost "that" much, really. Especially given they were pretty short, minor, clips for 30-year-old movies that pretty much everyone has seen.
Adam then goes on to say that since the earlier seasons of the show they've grown to testing more fictional things/ "myths." And, really, it's probably taken "that long" for them to really build the "rep" to be able to do this to make it an event episode. Had they done this in the earlier seasons it wouldn't have been quite as big of a deal.