I believe it’s more different factors all together than one single reason.
First you need to know that adventure games in Germany are more popular than in other countries in term of sales, but it’s not outstanding, if an adventure game sells 10K units full price on PC in Germany it will be considered as a success in 2012 (and it will sell 25K units lifetime). We could sell 30K units at full price back in 2003-2004 (100K+ units lifetime). so overall the sales of one adventure game in Germany, even a very famous one, lifetime, will not reach 100K units today, meaning that the sales in retail decreased there just like in any other territories, France, UK, or US for the last 10 years.
Germany still sells more units than any other country in Europe, whatever the genre, Farming Simulator alone sells in Germany what it sells worlwide on PC for example.
The differences are still here, but there are only partly specific to the adventure genre:
1. Germany is still traditionalist, there are less sales in Germany in Digital that in other countries in term of % compared to overall sales, Germans are still buying paper press, while it disappeared everywhere else and trust what the journalist write, it’s decreasing but slower than anywhere else.
This taste for old-school industry is also valid for the genre, Germany is producing Simulators, adventures, tactical RPG, genre that are not that popular anymore but they work pretty well on the domestic market
2. Adventure Games in 2D cost little money to be produced, we are speaking about 100-400K euros here, compared to 2ME+ for the testament of Sherlock Holmes or 20MUSD+ for LA noire.
3. the local Publishers and local Journalist are working hand in hand most of the time, it means, journalist and publishers discuss and compromise about previews and reviews,rankings, etc..it could be qualified as conflict of interest or corruption in other countries, but in Germany this art of dialog is really helping the local industry, this is pretty unique.
4. Germany has 90Millions inhabitants and the 4th ww economy, it has a big domestic market, one localization is easily recoupable when you sell 10K units
5. Adventure games makes it rarely on consoles, the lesser the budget, the more point and click it is, the less chance it has to be on consoles, therefore the console countries (UK, Japan, US) where console sales are so much higher than PC, are disqualified as country producing adventure games. Germany was and still is a PC country.
6. Strategy Games was they key genre in Germany in the last ‘90s mid 2000’, but those titles cost far more to be developed today than 10 years ago,and the sales decreased, meaning that the local publishers had to find another genre to produce in Germany.
7. everyone else in the world dropped adventures, leaving the door open for a country to become the leader, but instead of a country you can really count one publisher, Daedelic, the others are mostly taking advantage of Daedelic works in term of marketing, which is excellent by the way, it’s young, it’s a little underground, not taking itself too seriously…
Germany is also where most boardgames are created nowadays. I think they are simply a more cerebral audience.