Computerized political and racial gerrymandering places all of the voters who would tend to oppose an incumbent into as few districts as possible. The incumbent also tends to have a fund raising advantage since they can give favors to supporters who will provide them with campaign funds.
Incumbents win over 98% of the time
The congressional franking privilege allows incumbents to flood their districts with mail that often is little more than taxpayer-funded campaign literature. Large administrative and political staffs on Capitol Hill and in district offices attend to the needs of voters, all the while stressing the qualities of their bosses. Incumbents also receive taxpayer-subsidized travel, easy access to the media and, most recently, Web sites to communicate with the electorate. And they have the power to deliver pork barrel spending to their districts. The limits on all of those advantages are set by their beneficiaries -- the Congress members themselves.