Campaign finance regulations are an incumbent’s best friend. The incumbent already has name recognition, and a deep network of fundraising contacts. Heck, Congress’ franking privilege allows incumbents to send out de facto campaign messages for free. Challengers have none of those advantages.
It takes a lot of money to buy enough ads to get a challenger’s name recognition anywhere near the incumbent’s. Campaign finance regulations make it harder to raise that money, and harder to put up a fight against established officeholders. No wonder so many incumbents from both parties favor strict campaign finance regulations! It’s good for their job security.