Well, why was Greek and Latin culture so influential throughout all of Europe? I think the most compelling explanation for both of these is that they were literary cultures in the Ancient period. For generations Japanese scholars were learning Chinese to study the ancient classics, all of which were written in Chinese long before Japanese writing had even been invented. And they didn't start using Chinese characters and loanwords necessarily because they just loved Chinese culture so much - it was because all the ancient and prestigious texts used those words and characters, the same as we write with the Roman alphabet and use plenty of ancient Greek loanwords in English.