The growth boom in the Scandinavian countries was the industrialization between WW2 and 1980's. Living standards soared as the countries transformed from farming societies to industrial nations. As that happens, it's rather easy for any government to remain unchallenged, and in the case of Scandinavia the governments that took credit and remained in power were social democratic. Whether the boom was caused or merely correlated with the rise of social democracy is harder to say. History could have been different, we could have had a more liberal form of capitalism and we likely could have had even faster growth in these decades. Who knows. But to say that e.g. Sweden was more free market in the 60s/70s/80s than now I'd argue is not correct.

Liberalization of the market economies in Scandinavia has been taking place in a pace rarely seen anywhere else. Go to Sweden and find voucher schools for all ages, free taxi markets etc, - things you can barely find the US!