Answer:
Incumbents have several advantages over opponents in elections. First, they simply have better name recognition, people have heard of them. They often have more campaign funds. They also have credit claiming opportunities. If they've benefitted their constituencies by getting a project done, they can claim credit for that. Or they can claim credit for casework, doing favors for individuals like cutting through bureaucratic red tape. Some political theorists also believe they run against weaker opponents, because stronger opponents don't want to run against them.