There is not more prehistoric art in Spain and France.
This is because prehistoric studies were started on France, plus add a language barrier and a huge archeological search bias.
There is plenty of prehistoric art in the Balkans, as well as Morocco, Libya... and other non-developed, poor, second-tier language countries in southern Europe that have no funding for historical research, nor to pay for translations for you to read in English. Italy and Greece have as much prehistoric art as France and Spain, but they get less butget for it because other historical periods are preferred to get tourists atracted.
All these countries have caves. Caves are everywhere, it's not a preservation bias, although certain caves preserve better than others and have a local bias, it's not something exclusive of Spanish and French regions.
Also, it's not a prehistoric cultural thing, there was nothing different in Upper Paleolithic Cantabria compared with say, Romania, both were of Aurignacian culture.