Presumably it is because that particular legend is a quite local one and mostly restricted to Southern Bavaria and the adjoining parts of Austria.
The same goes for the Kyffhäuser legend and the former GDR. Plus: the young people don't know these legends anymore and the old ones have no real occasion to tell them. I mean, which family sits at home with the TV switched off, listening to their grandparents telling legends, myths and creepy stories?
The theme was also used in Paul Keller's "das letzte Märchen", a comperatively unknown novel. It was first published in 1905 and is rather pro-war, empire and very strict government. In his story it's Rübezahl who sleeps there. http://gutenberg.spiegel.de/buch/1433/17
(scroll down to the middle)
I like the novel because the language is beautiful and most of the story is quite funny (particularly chapter 3), yet the political undercurrents make me wince. It's interesting (and creepy for a rather left-wing social democrate) how in hindsight the 3rd Reich quite clearly announced itself for a very long time and how many different factors came together, making the result almost inevitable.