The main reason for this is that the use of money to get out political messages is protected by the First Amendment, at least according to the Supreme Court of the United States.
Congress has, from time to time, tried to enact laws that would reduce the power of money in political campaigns. However, these laws (like the recent McCain-Feingold law) have typically been struck down by the Supreme Court. The Court has ruled that people (and even corporations) have the right to use their money to get their political ideas out before the public.
For this reason, it is very hard to limit the influence of money in political campaigns.
Part of the reason it is so difficult to obtain a full hold on the beast of campaign spending because it is so heavily rooted in Constitution ideals. One of the most basic elements of the Constitution exists in popular sovereignty, ensuring that people have a voice in their government. At the same time, the political expression that is a part of the First Amendment allows individuals to petition their government, which can extend into financially committing to candidates that help enhance the beliefs of a specific individual. Corporations, political action committees, and public advocacy groups were not something that the framers envisioned, which means that these entities were able to use the Constitution to insulate themselves in their own pursuits of advocacy. The growth of economics as such a defining force in political life was also something that was not fully envisioned by the framers. Campaign finance law and reform has not been able to keep pace with how individual influence on the political process presents itself. It seems that the best that one can do would be to try to limit the egregious violations and live with a "decent" solution as opposed to an ideal one.