We were talking about this at work today, actually, because someone pointed out that 90,000 people died in the Sichuan earthquake and it barely made the 10 O'Clock news after the initial quake. The floods in Pakistan are generally considered to be the worst natural disaster of our generation, but had no where near the coverage of the 2004 Tsunami.
Our thoughts were that people have a particular fear about tsunami, maybe from horror movies. The almost anthropomorphism of the tsunami wave chasing us, and that horrible feeling of impending doom (the clip from 2004 of the person on the beach frozen solid in the face of the oncoming wave still haunts me a little). And perhaps there's also that bit of 'Western centricism'...we identify more with Japanese culture, being affluent, and can 'understand' it better, perhaps? There's also the element of 'sudden' and quick devastation - I think the death toll for the Pakistan floods was in the region of 1500, but over 14 million people were affected...but perhaps this isn't as 'shocking' as dramatic Tsunami footage, which has killed tens of thousands instantly?