Two reasons, Matt.
1) Safety. 2-Handed weapons, as constructed in our game, are not safe to use in a single hand. They tend to range from about 5-6 feet long and the level of control necessary for safe use simply doesn't exist with a single hand.
2) Simplicity. A system where damage changed mid-fight due to hand placement is rather complicated and confusing. It is confusing for people you are fighting against and it is much more likely to cause mistakes and holds. This is already notable in the current systems that exist which change damage mid-fight (backstabs and weakness spell, most notably), where targets often get confused by a sudden change in damage or the wielder of the weapon accidentally calls the wrong damage due to instinct. There would have to be significant pay-off in order to make adding another system of that type worth it, and given the safety issue, there clearly isn't.
-MS