I think it depends how you look at it, but night trains in Japan are actually not that popular -- so much so they had to reduce routes so the Sunrise Seto became the only surviving night train (as pointed out in your linked article), which why it's hard to book.

I actually wanted to book it from Osaka to Tokyo, but there were a number of hoops to jump through to book it from abroad with a JR Rail Pass that hadn't yet been exchanged, that I ended up just book another hotel night and then hopped on an morning Shinkansen to Tokyo.