I very much admire Duncan Hunter. Why is it so hard for a Congressman to get elected?
Several reasons. The primary one is that they serve a relatively small constituency, and so have no national, or even statewide, recognition. It also means that they only need appeal to a narrow segment to be elected, which often translates into lack of broader appeal needed to win nationally. Senators and Governors need to win approval on a statewide basis, and so tend to have more populist positions.
Senators tend to be centrist and Representatives tend to be more partisan to the Right or Left, depending on their district. The nature of the way the House conducts business tends to highlight this.
Then, there is the fact that there are 100 Senators and 50 Governors. Congressmen get lost in the crowd of 435. This means they have less influence on policy, and therefore less appeal to the money donors.