One reason that it would be very difficult to limit campaign spending is that the Supreme Court has pretty much said that campaign spending cannot be limited in any serious way. There can be limits on what is given to candidates, but you essentially can't limit what people spend on their own (not coordinating with the candidate).
In general, unlimited spending helps incumbents. People who spend money on campaigns generally want something in return (at the very least, they want access to power). The incumbents are the ones who have the power and are more likely to keep the power than the challengers are to win the power. So people will generally spend more money to help incumbents than to help first time challengers.