When you produce the world's most popular computer line, and you decide it is time to introduce its successor... just how badly can you fuck it up?
The whole Apple III story was a disaster. Rushed development and design-by-committee led to manufacturing difficulties, very high failure rates, constant overheating problems, limited backwards compatibility, poor expandability, and high cost. Only its operating system was truly a step up from the Apple II, but its modern features were ported back anyway. By the time they managed to iron out those kinks, the IBM PC had come out and taken over the niche the III was supposed to occupy.
The Apple III was soon discontinued, while the Apple II continued for a decade more.