New York is a popular target because it's arguably the most important city in the world, at least at the moment. It has strong signifigance not just to Americans, but to people in most countries whose livelyhoods depend on it. I think this is definatly changing, but it still rings true for the most part.
The US is not only the most powerful nation in the world, but has also been the biggest market in the world, and the most profitable. While other countries have surpassed us in some respects, we're still the marketplace of choice where people bring their goods before anyone else.
New York City has perhaps the most impressive natural harbour in the world, and is where a huge portion of goods entering the country go, for distribution and sale in other parts of the country.
If New York City goes down, so does it's marketplace and all of the infrastructure located there, not to mention things like Wall Street. We're talking chaos beyond the city itself and the USA, but an international crisis where national economies will collapse, fortunes will fall, and people will be scrambling to try and create alternative infrastructures out of nothing given that Wall Street is pretty much the #1 spot for international business administration.
A lot of people might not like this, and think it's "American Arrogance", but the point is that New York will resonate with people because it represents a lot to them as well as the US, and has for a long time. As the world changes, so will this, but that's where we are now.
In comparison other cities like LA, London, Bangkok, Paris or even Hong Kong are big deals and recognized internationally, and can be used in a similar capacity, but they tend not to resonate the same way New York does. Less people have a vested interest (globally speaking) in the survival of London Or Paris than in New York for the moment.
It also doesn't hurt that New York has a huge TV and film industry, right up there with Hollywood, and a pretty big infrastructure. That means it's very easy to use to develop visual media if you have the money. What's more it's important along with it's heavy use means that a lot of people are familiar with it, and familiarity helps generate the right feel when they see things destroyed.
