Keep the money away from the challengers and reelection is that much easier. Part of this campaign finance reform push is to force television stations to cut advertising rates for politicians during an election. Radio and television stations are private businesses. They basically have only one item to sell. That item is time.
Time has no shelf life.
Broadcast stations cannot manufacture and warehouse inventory to save for high-demand days. Every day, every hour they have a set, finite amount of inventory they can sell. In order to make a profit for their shareholders and employee profit-sharing plans these broadcasters have to get as much money for each and every advertising minute that they can. Putting it simply, if a law is passed forcing a broadcaster to give free or reduced-rate advertising to a political candidate,then that candidate is being allowed to simply confiscate a portion of that private businesses inventory to be used for his political purposes. It didn't stop there.