Reasons I can think of for that fixation are:

1. Germany's existing industrial capacity in terms of machining is a much closer match to hydrogen than to EVs. So in short it is wishful thinking mixed with a kind of self-preservation.

2. There seems to be a (somewhat unfounded) worry about energy storage when it comes to EVs, that many German technologists think is easier to handle and solve with hydrogen.

3. Germans culturally have a slight tendency to be fascinated by intricate and complex systems (which can also be a bad thing, see bureaucracy). Electric vehicles are conceptionally very simple, so the opposite. Hydrogen is a little bit more involved.

Toyota had a hydrogen fixation too: https://en.wikipedia.org/wiki/Toyota_Mirai

A more complete theory would encompass the Japanese drives as well.