It might be more accurate to track median incomes of Democrats and Republicans.
One possible reason for this stat: The most liberal states are on the coasts. The highest salaries are on the coasts. The highest costs of living are on the coasts.
I lived in metro NY for many years, as well as three less expensive states, and I can say with absolute conviction that earning $200K in NY or California or Massachusetts is probably like earning $100-$125K in many other places.
So sure, they earn $200K on paper, but are they getting $200K worth of value from it?