Maybe he's mad keen on football, but just dislikes the team captain of the team he's betting to win. 
Or maybe he's secretly devastated that he had to give up watching the FA Cup for Harry's wedding. I can hear the phonecall now:
William: "Harry, mate, you've been dating Meghan for ages. Couldn't you both have made up your minds to get married a few weeks earlier than you did? Then we could have had this done and dusted by April. I have a clear couple of weeks then. Now, thanks to your procrastination, I'm having to miss the FA CUP FINAL. It's the highlight of my year, Harry. You know that!"
Harry: [inaudible]
" Yes, I know attending my only brother's wedding is supposed to be more important. I said I'd come, didn't I? I'm just saying, it would have been nice not to have to choose."