Largely it is the system on several levels:
- Limited choice for voters. Who has the time, energy, and resources to campaign? Most people have a job and family they care about and takes up their time and is needed for secure income. Of that pool, how many are willing to step into the heavy, lying and misleading attack ad campaign system?
- Gerrymandering. Many districts are safe and only a primary challenger has a chance from heavily biased party loyalty in that district. Typically a primary challenger will have zero support because the Party takes care of its own as long as they are loyal. In this case loyal is to the president and party only. The attitude is best summed up by Rand Paul (R-KY) when commenting about the new Russian constituency in the Republican Party and their support for Trump, “It is a waste of time to investigate one of our own.”