When Java and friends were being designed, it looked like 16 bits would be adequate. They didn’t insist on UCS-2 over something better, they chose what looked like the best option at the time. It later turned out not to be. UTF-16 was created to fix that problem without having to completely redo their strings from scratch.

Likewise, C was created in a time when it looked like 7 or 8 bits would suffice. That turned out to be wrong. UTF-8 was invented to solve that problem without redoing C strings from scratch.