I think pretty much everyone has nailed it. It is to prevent other players from influencing the action. In live games you would get in trouble for influencing the action, but online is not regulated to such detail so most of the sites choose to block chat when it is most likely players will use chat in a way that is considered dishonest to the game. Sure, this can happen at any other point in the game, but a player's tournament life is only on the line when they are all in. Even if you have 1 chip you could theoretically make a comeback, but it's over once you're out, and that could cause the site more drama then they want to deal with by allowing chat when a player is all in.