14. "the most common reasons for incumbents retiring" In response to In response to 13
are redistricting (newly drawn district now less favorable for the general, incumbent forced to run against incumbent in primary, etc) and rough environment for upcoming midterms (likely to lose seat against other party).
excluding scandals and just outright betrayal of your party (relatively rare in both cases)...the advantage of incumbency is just firmly entrenched in voters when choosing party nominees.