I think the UI look and feel was very ugly for many users and that caused its demise. The cross platform skin was ugly. The native skins were in the uncanny valley.
The framework was reasonably good for its time. By the time good looking UI frameworks came, the bad reputation was already set.
Even the later JavaFX was a tasteless exercise. I opened some apps and you could tell within 1 second that something was wrong because all the text was using fugly non-platform-native (or somehow screwed up) text rendering.
The framework was reasonably good for its time. By the time good looking UI frameworks came, the bad reputation was already set.