because to be constantly good in a true team sport you need a lot of people practicing that activity (and in Norway they don't have that many volleyball players), meanwhile it's a lot easier once every while to find a pair working well together and establishing among the best in the world...
for what concerns the weather, we all know that all the best players are used to travel around the world the whole year, looking for the hot sun (even the Italian players always spend our winter/early spring in the Brazilian summer to find a better climate than in Italy...I mean, Italy...where we have 20 degrees or more around Christmas in the southern regions)...
so, that makes it a non-factor, after all (and still, they also train a lot indoor even in beach volley, especially the youngsters who don't travel the world so much as the professionals)...
p.s. by the way, it's full of European Countries with the same "problem" of Norway...look at Latvia, Austria, Czech Republic (mainly among the women), Switzerland (a few years ago they had 3 pairs fixed in the top 10 of the world ranking and won even Olympic medals)...the same Germany (recently they improved a lot also in indoor volleyball, but their men basically has always been competitive only in beach volley...and the women's results, despite being better than the men's, are also not comparable in terms of global success between indoor and beach...in favor of the outdoor version, of course)...