While Governors appoint a replacement for a Senator who leaves, they do not for the House.  A special election is held and must go through primaries into general, which usually takes 3 - 6 months.  A district who has a Representative step down, die, or otherwise leave is left without representation during that time period.  Most R corrupt reps did not seek re-election but very few D reps did the same.  This gives the R's a huge advantage when members are indicted and forced to step down.