I don't know about other states, but Tennessee election law is all about incumbent protection.  My suspicion is that what is true in Tennessee is probably true all around the country.  I'll describe how things are set up in Tennessee and maybe others can respond with descriptions of elections shenanigans in other states. 

   Tennessee is a relatively easy state in qualify to get on the ballot, but the qualifying deadline has been moved from May to early April.  The reason for moving it earlier was because most of the more controversial bills in the legislature don't come up until later in the session so they are now less likely to have someone challenge them because of public dissatisfaction with their votes.  If someone does decide to challenge them with a write-in candidacy they must get a certificate of write-in candidacy at least 50 days prior to the election.  If you don't have a certificate of write-in candidacy any write-in votes cast in one's name will not be counted.  So much for freedom of choice.  Some years the legislature is still voting on controversial issues after this 50 day deadline has passed which means if a candidate is unopposed in the primary there can be no write-in candidate to challenge him after that date and they are safe. 

   Here is the icing on the cake: if a party doesn't have anyone qualify before the deadline and someone decides to run as a write-in candidate for the nomination in the primary and he is unopposed he must receive votes from 5% of all of the registered voters in the district he is running in.  Until 2 years ago the number to be met was 5% of the voters that voted in the primary, a much lower threshold.  In the case of a congressional race that means the number of votes needed by the write-in candidated will likely exceed 20,000 votes, a tremendously high threshold for a primary considering that 1/2 or more of the registered voters may be in the other party, turnout is usually low for primaries, and that the candidate's name isn't on the ballot and voters must remember to write him in.  If one is unopposed and on the primary ballot they have no minimum threshold to meet.  Does there seem to be some inequity in this?  Why does an unopposed write-in candidate have such a high standard to meet, while an unopposed candidate on the ballot has no standard at all to meet?  Shouldn't they at least have the same standard to meet, which should be much easier for a candidate on the ballot than one that is not?  Such rules are great for incumbents, but limit the choices of voters.  It must be great to compete in a contest where you get to rig the rules to give yourself the greatest advantage, but that's politics in TN.  I should say that is Democrat politics in TN, as they were the ones that pushed these laws through the legislature.