It's interesting that the SPD only seems to have won Rhineland-Palatinate in 1998 but it's had an SPD Minister-President for the last thirty years now.
Structurally, Rhineland-Palatinate is fertile ground for the Christian Democrats: High rural population, most of them Catholic, dependency on agriculture, only few larger cities...
In 1991, the SPD won RLP after the CDU had governed there since 1947, mostly because of some CDU scandals. Ever since, a major factor in the Social Democrats' success on statewide level has been the incumbency of their Minister-Presidents (Kurt Beck 1994-2013, Malu Dreyer since 2013), who are folksy, popular and are just great fits for the state.
The fact that RLP is "naturally" more conservative is very well depicted in statewide polling: Between 2011 and 2016 as well as between 2016 and 2021, the CDU was the leading the Social Democrats in most polls, at some point in time by double digits. They just always blew it in the final weeks of campaigning.
Another point why the CDU did exceptionally well there (and almost won the state in 1998 in spite of the party's poor performance nationwide): Helmut Kohl is from RLP, and was a (relatively) popular Minister-President there before becoming Leader of the CDU.
