Looking at Area 51 site statistics, all the numbers are looking good but with one glaring exception: only 2.6 questions per day.
This is terrible! And that number does not even tell the whole story. The figure is artificially inflated by established users posting questions just to make the place look less lonely. Who knows how low it is in reality.
There are of course specific problems with german.SE and its culture, but these mainly affect whether people stick around and contribute regularly. Many Answers are of poor quality but enough Answers are sufficiently useful that, even in the absence of professional linguists and philologists, german.SE should have gained an Internet-wide reputation by now as a resource for (mainly) non-Germanophones.
Consider, however, that the problem may not be so much in the supply as in the demand. In August of this year, Germany introduced the "blue card", a scheme to attract skilled workers and qualified professionals from outside Europe to the country and to allow such people up to six months to go job-hunting in Germany. However, as this article in WELT Online explains, only a handful of blue cards have actually been requested and awarded. A staggering three (!) card recipients actually came to Germany on job search grounds.
The article hints at part of the reason:
"Deutschland wird als nicht so attraktiv wahrgenommen und ist auch wegen der Sprache schwierig", erklärt die Expertin. (...) Leider sei Deutschland im Ausland noch nicht berühmt für seine Willkommenskultur.
I don't think that many Germans actually understand how provincial and insular they come across to foreigners (and I am not talking about avowed xenophobes here). In my experience this is getting worse, not better. At the same time, Germans are neglecting or even discarding much of their cultural heritage as if it were nothing more than outdated fashion.
People who have options decide based on a variety of factors. Besides the absence or presence of a welcoming climate, a principal consideration is economic opportunity, and there simply is not as much dynamism in Germany as in the 1970s, 80s and 90s. And those who do find lucrative work in Germany can easily go for years without learning any German whatsoever.
Desire to become proficient in German is now mostly limited to people who take a genuine interest in German culture and German literature and we may have to simply accept that there are fewer and fewer of those around. This decline in demand parallels the sad demographic trends in German society (it is now the second-oldest in the world, after Japan's). If the nine-figure amounts pumped by the German government into the Goethe institutes worldwide and other cultural promotions are unable to reverse this trend, it is unreasonable to expect that german.SE can remain exempt.