The original Unicode marketing / position statement from 1988[1] may provide a clue:
“In the Unicode system, a simple unambiguous fixed-length character encoding is integrated into a coherent overall architecture for text processing.”
“Unicodes [sic] are the most straightforward multilingual generalization of ASCII codes: - Fixed length of character code (16 bits); [...]”
“Are 16 bits [...] sufficient to encode all characters of all the world’s scripts? [...] Yes.”
“[A] fixed length-encoding is flat-out simple, with all the blessings attendant upon that virtue.”
Etc., etc.
The hypothetical possibility of more than 2^16 characters was introduced in Unicode 2.0 (1996), while actual such characters didn’t appear until Unicode 3.0 (1999). Windows NT shipped in 1993, OpenStep in 1994, Java and JavaScript in 1995. UTF-8 was presented at USENIX in January 1993; a contemporary exposition[2] says that “the 4[!], 5 and 6 byte sequences are only there for political reasons” (presumably referring to the fact that Unicode committed to 2^16 code points while the new, Unicode-compatible draft of ISO 10646 stuck with 2^31).