Support documents on the three countries have messages saying that Cortana will be integrated into Microsoft 365 productivity apps, and as such support for the Android and iOS apps will be ending.
It’s not clear why Microsoft is specifically ending support for the three countries, but this can simply be the first of many rollouts, with different regions getting different shutdown dates. Right now Cortana will still be available for iOS users in the U.S.
Microsoft will be releasing an updated Microsoft Launcher after January 31, 2020 so the app should still work even with Cortana removed. The Microsoft assistant will go offline just after four years since it was introduced in 2015.