The unicode consortium got unicode wrong, not java. At the time Java was created, there only was a BMP and Unicode had a purely 16-bit design. Therefore, it was perfectly reasonable for java to assume a 16-bit char. Unicode was later extended to support more than 2^16 characters, but that was after java was created, hence the need for UCS-2/UTF-16

Other languages which strongly value backwards compatibility and were created around the same time have the same issue - e.g.: C#/.NET, i believe python as well