It doesn't even have to be a better/worse thing; there could have been a theme the people in charge were looking to illustrate and your paper didn't happen to fit. Too, they could have seen you contributed last year and wanted to give others a chance? I think it's kind of great you presented last year, when you were younger, etc. as that is more impressive than at the end when they might be throwing sops to those who have never presented yet but are graduating, etc.? It's like letting football players who have been sitting on the bench for three and a half years play in a game so they can say they played? You're already established. You won the Oscar last year, but, of course, were nominated this year.