The obvious answer is the Bank of Japan. Read here, here and here
From the weekly chart , you can see an obvious trend- ONE WAY UP for all major pairs trading against the YEN.
The answer is simple. You would want to buy the currencies that offer you a higher rate of returns as rewards thus USDJPY is the most preferred currency pair amongst all others.
On average, we are talking about a 50% return since March 2020. I believe so long as the US dollar remains strong and interest rates for next year continue to go up, YEN weakness remains. So, the party ain't over.