Final answer:
Congressional incumbents often win reelection due to franking privileges, substantial campaign contributions, and superior media access, which combined lead to an incumbency advantage.
Step-by-step explanation:
The possible reasons why congressional incumbents win nearly all of their reelection campaigns include franking privileges, significant campaign contributions, and easier media access. Franking allows incumbents to send mail to their constituents for free, which helps build a relationship with voters. They also tend to have more campaign funds than challengers, as donors prefer to support a proven winner. Easier media access grants incumbents more visibility, thereby increasing their name recognition. Additionally, incumbents have the benefit of a dedicated staff and institutional resources that can aid in constituent services, making them more effective at publicity. All these factors combine to create an incumbency advantage that challengers find difficult to overcome.