It is important that only the people that represent the majority are the ones that can donate to the candidate of their choice. If there is anyone that the candidate needs to impress or win over is the people of the United States of America and not Wall Street or any large corporation.
The country belongs the people and not anyone else, we decide who we want in office. We don't want our candidate to be conflicted with bribes and favors that jeopardize the tax payers.