1. Sources are often arguable if they are legit or accurate.
2. Elections are time sensitive. Ya only need to freeze a story temporarily to make it not worth anything regardless if it was accurate or not.
3. They would abuse it. It would do more harm than good.
Who validates the sources? who validates the validators? How many sources will be legally created just to be able to cite a source?
What you will also end up with is left vs right media constantly trying to discredit each others sources, just as much if not more than the political candidates themselves. What's the point in having a 'reliable' source if your opposition is able to constantly point out, that source isn't always reliable and has other dark secrets that may or may not be related to the political topic? What you can end up with is information sourcing citizens/companies being destroyed over politics, whether they are telling the truth or not. This can then lead to less people willing to do that job/work, which means less information in future elections, and only getting partial information can lead to even greater corruption, and usually does.