Like @timcliff said, the main problem is sybil attacks. There always has to be a way to keep people from abusing the system by simply creating many many accounts; one-share-one-vote is a very simple way to make sure that happens. Here's the issue: if an account worth $100,000 has the same influence as one with $500,000, then anybody with $500,000 can simply split his account into 5 pieces and get 5 times the influence.