This is my speculation based in my historical knowledge of Japan, which comes mostly from Samurai media and those East Asian Genocide games we call Musou.
Regular island countries are already very set in their ways and reticent to adopt foreign cultural customs.
Japan, on top of that, for over a thousand years was a generally isolationist country, with most of its contact with the outside world being the Mongol invasions, or in turn, the Japanese raids on China and Korea. Their cultural norms were extremely rigid, and as a civilization, they stayed almost static for all that time. They were in a state of constant civil war between the many clans (with the aforementioned lunch break to fight the Mongols), which left little room for social advancement. Until in the 1600's, one clan managed to unite Japan under one government, the Tokugawas. And while they were generally isolationists and culturally rigid before, Tokugawa made that into law. The country was almost completely closed to the rest of the world for almost 300 years, and any and all societal advance and social mobility were outlawed. You were born a farmer, you'd die a farmer, and so would your son, and your grandson, etc.
Until America happened, and Commander Matthew Perry (long before he became a TV star by playing Chandler Bing in FRIENDS), showed up in a warship and fucking bombed the capital: Open the country or I'll open it for you.
So a new civil war begun, and the westernization movement won.
Japan spent nearly all of its history almost unchanged culturally, a country that stayed medieval throughout the the Age of empires and the industrial revolutions, until they were dragged kicking and screaming into the larger world in the late 19th century. And then again after WW2 and the bombs, when America broke all their shit and rearranged it in the image of, well, America.
So all this history lesson is to say that, their rigid, medieval values are still in there, underneath the modern ones. Their loyalty to their clans and leaders and the rigid, stratified social structure now manifest in their corporate culture.
You know what's another pretty typical medieval value, one we can still see in African and Middle Eastern countries?
GET THESE GIRLS PREGNANT AS SOON AS POSSIBLE, WE NEED MORE CHILDREN TO REPLACE THE ONES WHO DIE EVERY DAY, WE NEED THE MANPOWER
And so, this society that's one of the most advanced in the world, but actually still has a lot of its medieval mindset in place (plus the trauma of getting completely humiliated and dominated by America over and over), has had a lot of trouble letting go of its attraction to young girls.
Couple that with their obsession with appearances, their sexual repression, and the cultural need to fit in and not stand out, and you'll get really psychologically fucked up people, with fucking weird kinks.
They only outlawed some forms of child porn in the last couple decades.
You know that mangaka, the Rurouni Kenshin one, that got caught with CP magazines and DVDs and got pretty much a slap on the wrist and a small fine? You know why that was? His crime wasn't having the stuff: it was not getting rid of it within the legally allowed period, and getting caught trying to get rid of it after it.
In contrast, getting caught with regular ass weed will get you pretty much unpersoned and disappeared by the government, because historically they hate drugs, because the Chinese smuggled opium into Japan back then, so drugs are evil foreigner shit, as opposed to our proud national tradition of child pornography.
That's Japanese law for you.
TL;DR: I think the Nips fascination with lolis is because they're deep down still a medieval society trying to pretend they're up to date with the rest of the world.
