A Dave Barry column from 22 years ago[1], describing how 3.5-gallon-per-flush toilets were banned in the US, makes me wonder if some kind of import restriction or regulation about what can be installed in certain places applies. Though there are people in this thread saying they've got one in the US, so that's a point against that theory.

[1] https://www.baltimoresun.com/news/bs-xpm-1998-11-01-19983051...