Well thank you Diver.
It was good of you to bring up the turn on/off times of the components, but i think there is a much simpler explanation about the 5.5uH inductor. The minimum Ton time of the chip is spec'd at 19us, and with that much time and a 12v input voltage at the end of the Ton period the current in the inductor would have reached something like 40 amps, and we know that the transistor will never be able to handle that kind of current as it is limited to 1/2 amp. As a matter of fact, due to the input voltage and min Ton time the minimum inductance for this app would be about 456uH. Any lower than that and the current demand exceeds the internal transistors rating. Make sense to you?