There is a commandment to obliterate Amalek.
Shaul Hamelech was commanded to obliterate Amalek and failed.
Dovid continued in the process, but obviously didn't destroy them completely.
None of the later kings of Israel (including Chizkiyahu who was a righteous king, Shlomo who was a world power etc.) even went to war against them.
Why was this commandment never fulfilled?