great question. remember the earthquake in Bam, Iran? what about the genocide in Darfour, which has probably killed an equal number of people. i think two things are going on here:
1. this was nature -- sadly, in response to things like genocide, it's easy to say, "well, that's sad, but that's what goes on in these places." with a tsunami, and the resulting spectacular home video (as opposed to an earthquake where footage is pretty much unattainable), there's a feeling that we're simply gnats on the earth's crust and we can be swatted by nature at will, there's a level of existential angst that i think is very palpable and makes the story that much more compelling -- any Westerner can relate to that on a philosophical level. there's no enemy here, no bad guys to explain the situation and draw lines of understanding.
2. a huge number of the victims were children, and it's utterly wrenching to see the parent-child bond (that's pretty universal) severed in the most brutal way possible. parents screaming over the bloated body of a 6 year old, or shell shocked new orphans with dead looks in their eyes trying to understand what's going on -- even my mother, who's pushing 60 and has already raised 3 kids, was inquiring about adoption.
at the end of the day, i don't think it's because there are so many westerners, at least from an American perspective. there were very, very few Americans washed out to sea (as opposed to, say, Swedes), and while i think we feel European tragedies on a slightly more personal level due to shared Western values and heritage (to a degree), i don't feel as if that has anything to do with this situation. even if there weren't a single Westerner -- and, let's be honest, that's really a euphamism for Caucasian, and we're dancing around what's at the heart of this question, which is race -- killed in the disaster, i think you'd see much the same reaction.