I don't know the answer to that question. I think the reason is as varied as people are varied. AI has been unable to obtain use rights to any Beatles songs, for example. Why? Who knows. Maybe Michael Jackson, who owns them these days, wanted more money than AI was willing to pay. Or maybe Paul McCartney and Ringo Starr, the two surviving Beatles, had a veto over their use. We've all noticed that Rod Stewart is not letting them use his original material, but only the great standards Stewart, himself, has covered in his recent albums.
Stewart is only one of hundreds of singers who have covered those great songs through the years. So, in my opinion, this is not really a Rod Stewart night. It's really a "great American songwriters" night.
As for use of a song on AI being good advertising, maybe. But look what happened on country night. Chris sang Keith Urban's smash hit, "Making Memories Of Us," and in his critique of Chris' performance, Simon said on national TV that it isn't a good song. How the heck do you think that made Urban and the songwriters feel when they heard that! And do you think they will want to let AI use any more of their material?
They are going to do a "songs of former American Idols" night. Simon has often slammed former AI winners and runners up. I can easily imagine him telling one of this year's favored contestants that they sang a song better than the original.
If I owned some highly successful songs, I sure as heck would not want them performed on AI where an arrogant Brit might slam them or me just for a dramatic moment that would keep the buzz about the show going.
What's the saying about laughing all the way to the bank? :)
But, how long do they get the rights to perform the song for? Do they have to get the rights again next year if someone next wants to sing it?