Absolute speculation on my part (with a few data points that give rise to the speculation).
I am guessing that they are not writing songs in English.
The experience from the Eurovision Song Contest is that to be successful outside of the contest, the band(s) must write songs in English. And thinking about that for a minute, about 25% of the world speak English as a result of British colonialism. Amongst non-native English speakers, English is the most common second language. Learning a second language adds less than 10% to lifetime earnings unless that second language is English in which case it is about a 25% increase in lifetime earnings. English is the international language of commerce and science (and probably many other things of which I am unaware). So, I speculate that to be successful in music you need to have English songs. Notable exceptions would include Rammstein (and can't think of any others off the top of my head).