With amiga owner track record with upgrades, commodore should have realised most people would go with the 1200 and given it a better sized trapdoor. they gave the slot some pretty impressive capabilities but not enough space in the trapdoor to fit a device to use them all.
Imagine a card with 060, two simm slots, clock, pentium pc emu or ppc, fast buffered ide or scsi, sound chip, and rtg video. the slot could handle it (with a psu upgrade) but there is noway you would fit anything like that in there.
at the time i wondered why noone built an almost flat case filled with goodies that the 1200 could sit on top of, that would connect with a cable throught the trapdoor.