What factors work together to limit competition for seats in congress?

Answer 1

The biggest factor is "gerrymandering", or drawing congressional districts in bizarre shapes to favor one party over another. The term comes from the name Elbridge Gerry who as Governor of Massachusetts drew a congressional district to protect his political power that vaguely resembled the shape of a lizard or salamander. A local newspaper called it a "gerry-mander", and the name has been used ever since.

In the modern era, congressional districts are drawn to create safe districts for the political party that controls the re-districting process in the state. This process packs each district with voters of the preferred party, concentrating all of the voters of the opposition party to a smaller number of districts. The average congressional representative is more likely to die in office than to lose his next election.