Not only does the lack of competitive districts protect incumbents, it also contributes to the ideological gulf we are seeing in Washington. Basically, most incumbents in the House retain their seats by appealing to the activist voters in their district — either Democrats or Republicans. For most of these lawmakers, there is no incentive to reach across the aisle or talk about the need to compromise.
Changes in population reported by the Census drive the redrawing of district lines. The party in power gets to do the drawing, resulting in districts that favor its candidates.
Unless that practice changes, gerrymandering will continue to be a tool of politicians, to the detriment of those they are elected to serve.