I think it was because a the game grew, and the number of playfields grew with it, it was not a seamless transition anymore. It became very bad towards the end before it was removed/changed. At least, that's how my increasingly failing memory remembers it. Edit Even if that wasn't the official reason for its removal it was still very much a fact. It was laggy and not seamless anymore.
Doesn't seem much different in practice from the warp drive effect. It might be worth bringing this STO transition effect back in a newer, sexier form.
