In a 2p game, you know that both you and your opponent have a Province in hand. You play Tournament, and neither of you reveal your Provinces - and this was the correct strategic move for both of you. Why?
Well it's easy enough for "you" -- you're far enough behind that you don't want to gain the last Duchy and lose on piles. Or you want to keep Province in your hand for something else -- discard for benefit, Crossroads, whatever.
But for your opponent not to block your Tournament? Maybe he hopes you will mistakenly end the game on piles. Maybe he wants to hide the fact that he has a Province in his hand. Say he knows that you have KC-Tournament-Tournament-(other action) in your hand and thinks that you're testing him with the first Tournament.
Or, you only have one Tournament in your deck and your draw pile is empty. He wants your Tournament to miss the reshuffle.
Oh, easy alternative answer -- your opponent is possessing you, so he doesn't reveal his own Province. You don't reveal your Province either because your opponent doesn't want to lose on piles, or wants you to have the Province in hand for something else (see above).