The simple answer is 911 and the Tsunami aren't comparable. They just aren't.
One is a natural distaster and the other is a attack. One is a horribly tragedy that was unavoidable and yet will most likely happen again in the future. The other was attack by terrorists on a high profile target. While tragedic, its also noted they were attacks.
Think about it, besides people from Iran - how many people remember the earthquake on June 1990, that killed 50,000
Or what about this earth quake that happened in 1976. How many people actually know and remember these things.
China?
Ok now considering those two earthquakes results, think about it. How many people know about them or had heard about them. Now how many people know about Pearl Harbor, Oklahoma City, When Kennedy was Shot? Now those things are more focused towards Americans. So how about Chornobyl? The death tolls were far less than say the tsunami or the other two natural disasters I listed, Yet why do people remember them and not the others.
The simple fact is natural distasters aren't etched into history the same way that human events (attacks/mistakes) are. They just aren't.
So you cannot compare them on which is worse. Yes the death toll is far worse in the tsunami, but that is just a numbers game, and frankly, the purpose of 911 wasn't to just kill people. So how can you compare those two things? You can't.