PoohBear, it's because Smalltalk, Oberon, and J2ME systems didn't need MMUs. People came up with MMUs because of low-level languages, a category which includes assembly and C. But we're just rehearsing debates that are half a century old.
(On the other hand, the B5000 had hardware memory protection despite being programmed in Algol. The B5000 inspired Smalltalk, which inspired Oberon and Java. But its memory protection didn't use an MMU.)
I was a user at the time, and also a reader of many computing magazines.
GUI environments of the 80s all brought multitasking with them, and system stability was mediocre to very bad... All writers pointed to memory protection as the cure of all this. See also Mac os history for a more detailed usecase.
Rising software size and complexity made the industry abandon assembly programming for higher level languages and for GUI apps this quickly meant C/C++.
(On the other hand, the B5000 had hardware memory protection despite being programmed in Algol. The B5000 inspired Smalltalk, which inspired Oberon and Java. But its memory protection didn't use an MMU.)