The only Democratic pick up from 2006 that was lost in 2010 was Ohio. The only Republican pick up from 2014 that was lost in 2018 was Illinois. 1994 is the worst offender, with five 1990 Dem pick ups turned into single-track Governors.
Why do only those count and not incumbent governors, open seats, etc.? I think it’s pretty obvious that Walker wouldn’t have lost in a Clinton midterm, for instance.
I don't think that's obvious.
What makes so-called "wave" elections so impactful is the ability to wash out entrenched incumbents. You don't see that as much with gubernatorial elections because most voters can judge their governor on his or her own merits running the state, and with federal races that becomes much more complicated.
Gubernatorial elections tend to cycle more similarly to presidential elections, such that after being in power for a certain time, a state may go for something new. This is why candidates like Tim Pawlenty or Jodi Rell were able to win in 2006, but be replaced by the opposite party in 2010, despite national federal trends going the opposite way.
The fact that so few gubernatorial incumbents lose in so-called wave elections compared to their federal counterparts suggests that there are other dynamics at play. There's a lot of evidence that most voters can appreciate a Governor of an otherwise rejected political party at the federal level. States still have their natural leans, which is why incumbents defeated us a good metric to analyze.