Carl Weissner started the Bukowski-hype when he published its first translation at tiny Maro-Verlag, then the big editors discerned that they could make money with Bukowski, and it all began. Germany was a country of readers in these times and book shops flourished.
Bukowski once mentioned that writing with Carl had been kind of spiritual relief for him, and in later years he hadn't such a conversational friend any more, Carl had been the only one, whatever that means, and Carl once mentioned that he really focused on these letters.
we should keep in mind, too that Bukowski was a commercial success in all European countries, and Carl made german-only translations, though he was probably kind of European representative.
Bukowski was a master of style, and his style creates a certain unique atmosphere, and this is very very hard to translate, especially when the environment is different in other countries.