Before radio and TV, only the rich elite who had the time to research politics voted. After radio and TV, only the rich elite who had the money to buy election ad slots got representation.
Edit: These numbers are from 2010 congressional elections. Also it's illegal for federal elections. State elections have their own rules.
Additionally, wouldn't it be better to look at expenditures in later elections such as the 2012 presidential election? Using the 2010 data is a bit biased because the Citizens United decision was only ruled on in 2010.
As for your point of donations being illegal for federal elections, it is true that direct donations are regulated, however outside "independent" spending by corporations is not illegal.
Additionally, Super PACs can accept unlimited contributions from corporations as long as they use the money for independent expenditures.
To answer your question, there are probably many reasons, but one reason is because corporations are paying politicians. Here is a story from The Guardian that explains the payoff http://www.theguardian.com/business/2015/may/27/corporations...
Also, here is another story from the Huffington Post that describes the corporate influence on the TPP. http://www.huffingtonpost.com/dave-johnson/now-we-know-why-h...
One last comment, while it is indeed up to our politicians to vote yay or nay on this, it is absolutely up to the people to keep our politicians accountable. Right now, the American people are for more concerned about feelings and witty social media posts to care about the TPP. When Americans stop caring about politicians and stop holding politicians accountable, it's quite easy for them to pull something like the TPP.