> how come Impeller still wasn't the default engine for all platforms

Because Impeller has had quite a bumpy ride. Even as recently as June I reported a bug/regression with basic stuff (lines with partial transparency on desktop). All my Android apps are still built with --no-enable-impeller because I've had many instances where the app just shows a blank screen due to driver conflicts.

It may be getting close to stable now, but it wasn't a smooth transition at all.