I think because in retrospect it was obvious and with a bit more care and thought about it I could have figured that out. My problem has always been that I fly past things too quickly. Ready, Fire, Aim as my parents would say.
It might make you feel better to re-read the first two problems, because the second one makes very clear that Smullyan didn't expect you to just know the "No native can claim to be a knave" rule off the cuff.:-)