UTF16 was an extension of the original "obvious" Unicode encoding, back when Unicode started and was defined as fitting the world's languages into a 16-bit spec (what is now the Basic Multilingual Plane). UTF16 allowed (most) older UCS2 documents to be upgraded for free to UTF16, much the way that ASCII documents are also valid UTF8.

Some of the weirdness in the Unicode spec even comes from the need for backwards compatibility. 17 planes and 1,114,078 total usable codepoints... these numbers would not have been arrived if the system had the foresight that 16-bit wasn't enough in the first place. They were derived from reassigning private use area codepoints into surrogate pairs for UTF16. Unicode would probably have rather (maybe should have) started out as a 32-bit spec and avoided this mess from the get-go.