Because the Apple Adjustable Keyboard is actually two physical ADB
keyboards, and one interferes with the other on the same ADB bus (only
the Mac IIfx had two ADB microcontrollers and, hence, two ADB buses on
the one system).
Many people have complained that this prevented from playing some
games on the Macintosh, where key presses on the numeric keypad (one
of the two physical keyboards) would "cancel" any key-down events of
modifier keys on the main keyboard. Because of this massive
disturbance between the two keyboards on the one bus, many games were
rendered unplayable on the Mac.
The Apple IIGS would have the same issues, considering it used the
same ADB microcontroller as the Mac SE and Mac SE/30. And the
problems continue to plague the Power Macintosh 7500/100 and Macintosh
LC 575, since I happen to have both machines, and the aforementioned
keyboard.
;-)
--tonza