iPhone has 51% market share in Japan across all age group[1] (and even as high as 84.8% in some demographic[2]). From my 5 years of living here, I’ve never seen anyone use iMessage even once. The dominant messaging app is LINE.
Seems like LINE became the predominant app way back in 2012 (it released even before iMessage did in 2011), so my guess is that it took hold before iMessage ever had a chance, unlike in the US where texting was always the main way to communicate on phones up until iMessage integrated texting to absorb all those users.
[1]: https://www.idc.com/getdoc.jsp?containerId=prAP51933524
[2]: https://japantoday.com/category/tech/study-shows-iphone-the-...