The answer to your question is multiple victory conditions. You may be winning on the map, but an opponent may be pursuing a quest victory. Even within the realm of a conquest victory, you may control half the map, but maybe your opponent has been pursuing a special friendship with a dragon that is going to wade thru you armies. Or perhaps the channeler himself has become a Sauron-like figure able to decimate your troops in combat.

I'd also suggest you have a read of this interview with Brad. He details some interesting ideas concerning family ties within the game.

http://www.shacknews.com/featuredarticle.x?id=1201

I really have to disagree with you on this. I once won a game of risk after only having two teritories left in the European continent. After some turns building up my army again, I was able to win the game because I had to wipe out a player. That player was getting beaten up by one of the others had was at the end in almost the same situation as me, so with a few lucky rolls and within one or two turns I could finish of that player and won the game.

Now, to answer your question, as some of the others already mentioned, I think the various victory conditions will make sure that there is always a chance to come back from a seemingly hopeless situation. Now, from the beta it looks like it will be possible to pick and choose which victory conditions will be available, so if you only enable the conquest one, you might still feel unbeatable after conquering half the map.

I remember a four hour game of Risk that came down to a die roll - one attacker, one defender.  The defender was on his last region but had a ton of cards ready to play, the attacker had gone all out attack and almost every territory was defended by only one soldier.  The attacker won, got the cards, and the other player in the game, who had a lot of territory but didn't have a set, immediately resigned.

So Risk isn't always boring...