It's the normal popularity cycle. Names go in and out of fashion. Most names are only in the top 20 for 20-30 years and then fall again. This site only has the top 20 but it was probably in the top 100 for a while after that.
Dieter is quite unfashionable in Germany now and it has become a bit of a joke name. Maybe because of
Dieter Bohlen, but not sure. It is just seen as THE ultimate unfashionable name, maybe sort of like
Bertha or
Beulah in the US.
Will it come back into style? Maybe.
Erna was a bit of a joke name for a while but is getting more common.
Please rate my list:
https://www.behindthename.com/pnl/6232