He had a very close call in 1998 against Mark Baker (51.56%-48.44%) After that Illinois 17 was gerrymandered into the rabbit on a skateboard format that made it considerably more Democratic. He consistently got reelected after that fairly easily.
Part of his appeal is that he was a Vietnam veteran and was very involved in veterans affairs. That probably made him more appealing to Western Illinois. Also that part of the state has swung very Republican only in the past couple of years. It was 'moderately' GOP in the years Evans was in the house.