They have a uniquely high level of internal debtors, the government bonds were bought by Japanese people much more so than other citizens buying their countries' bonds. I didn't understand why this made a difference though.
That makes it unattractive for speculators to bet against the country. Countries like China or Germany are in a similar position.
Japan currently funds more than half of their gov't via borrowing, with the demographic shift and the internal demand for JGB turning towards net negative, they will increasingly rely on external buyers of their debt. I believe China has been a buyer of late.
What has worked for Japan historically may not work so well going forward.
In a sense, this system of internal borrowing in a hidden tax. While it of course is not mandated, it's required in order to keep things going smoothly.