Is there some reason that solar power industry investments would have to be financed using debt rather than equity?
To go into opinion, I'm heavily leaning towards the "ideology" explanation. The current libertarian finance minister is rather publicly operating on a platform of "the government handling money = bad".
Yes, it's that. The usual slogan for that can be roughly translated "The government is not the better entrepreneur" ("Die Politik ist nicht der bessere Unternehmer"). It's usually trotted out by conservatives and libertarians in Germany if someone dares to suggest that the government could own some company or even only parts of it.