> Many of the incumbents "chose" not to run again. Why?

Never Trumpers. 2018 was a house cleaning for the Republican party. When Trump became president the party was split between the new school Trump republicans and the old school Neocons. The Neocons thought they could beat Trump and take their party back. They were wrong.

After Impeachment a lot of those democrats stand to be replaced with pro-Trump Republicans and the party will be better than ever.