One reason is that Finland is just about the whitest of the rich white countries. It only recently hopped on the Third World refugee freight train to who knows where. (White American kids would lead all the countries in the world on the PIRLS, unless Asian-Americans had their own country.)
But I guess we should also be taking seriously Finland's laidback educational techniques, which are, on the whole, the opposite of the more obvious way the northeast Asians grind out high test scores.
By the way, the outstanding Russian performance on these new tests (as opposed to the middling Russian performance on the PISA) is ... interesting.
There might be a niche occupation in the future for somebody to be the Unbiased Expert on international testing.