Because you don't have a bouquet to hide your beer belly.
While its not expected of you to bring a 6-pack to your wedding, you don't want to be bringing a keg either! Get on board, wouldn't you love to steal the attention from your bride, because Mr. Right could also be Mr. Ripped too? I already am, but lucky for you, she already said yes to you! (oh, I'm just kidding, but not about the 6-pack).
The whole point here is that it is just as much the biggest day of your life, as it is hers. Pay some careful attention to yourself, when you look your best, you feel your best.
Here's another slant. I think you're going to enjoy working out a lot more than having to help choose between 3 shades of pink table clothes. Hey, at least you're still contributing.