Actually I'm not sure it doesn't. In America the Dresden bombing is better known than Guernica, perhaps in part due to Vonnegut's Slaughterhouse Five.  Dresden is far more part of the German consciousness than Guernica is, or for that matter the German destruction of various cities during WWII. In Poland Dresden is also better known than Guernica.  On the other hand it makes sense for Guernica to be a larger part of Spanish collective memory than Dresden - it was the Spanish who were its victims (or Basques I guess if one wishes to quibble)