MP3: "On 7 July 1994, the Fraunhofer Society released the first software MP3 encoder, called l3enc." [0]
Opus 1.3: "libopus 1.3 was released on October 18, 2018" [1]
I feel like this is like asking "Why didn't Intel realize how flawed 8086 processor was, the Mac M1 makes the x86 look like a joke."
MP3 is so much older than Opus 1.3 hardly a fair comparison to expect 1991 tech to anticipate 2018's music styles, dynamic range, processing power, etc...