Without comparing the magnitude of the disasters or their emotional impact, 9/11 was due to a theoretically preventable cause, directly related to scheduled airline flights. The U.S. and other governments are taking steps (also without getting into how well planned or executed those steps are) to prevent a recurrence. The nature of the event and those steps have had, and continue to have, a dampening effect on air travel independently of what else might be going on in the economy.

The recent tsunami was a natural event which is unlikely to recur for decades or longer, which had no connection to air travel, and that nobody can do much to prevent in any case. (We can do a lot to reduce the damage resulting from a recurrence, but that's not the same thing.) Any impact it might have had on air travel is indirect, based on the desirability of going to the affected regions.

There is no reason the effect of the two disasters on air travel should be similar.