Very few people used Apple II Desktop back then. To really be useful, you'd need a hard disk, which wasn't that common, and ProDOS software, which wasn't common either. ProDOS is also a single-task OS and Apple II software expect to have the full machine under its control (not sure Apple II Desktop has interfaces to build GUI apps for it - would be fun). All that ProDOS provides is a hook you call when the app exits so that a program launcher can be reloaded (which is, IIRC, what Apple II Desktop does).
