It was basically sort of like the web but for mobile Java - a whole bunch of not entirely compatible and uniquely buggy implementations, to which adding feature

It was basically sort of like the web but for mobile Java - a whole bunch of not entirely compatible and uniquely buggy implementations, to which adding features was a very slow committee driven process meaning every new feature got reimplemented in 4 different incompatible APIs first. Additionally at the time the mobile makers were totally in hock to carriers, who usually insisted on having their own app stores with totally developer hostile processes. So to even run on a lot of phones was a huge PITA because you had to get every carrier to sign the "midlet" and they had no easy process for this. So the issue was not so much Java the tech - Android proved that - but more the model around how it was managed. Android took the basic concept of J2ME, dropped the actual APIs and design-by-committee (triggering the lawsuit with Sun/Oracle in the process), created a robust open source implementation, gave it away for free and invested heavily in their app store tech. And that led to success.