Here are a few reasons I have thought of:
1. Large Jewish population
2. Numerous military installations.
3. Numerous tourist attractions
4. President's brother is Governor.
5. Site of controversy that ultimately put President Bush into office.
Additionally, we already know the terrorists were operating within a 10 mile radius of the latest anthrax incidents.
Bin Laden seems to be living in this fantasy world of his own definition. Now that he will have to make decisions with people watching, that is to say, with real people opposing him, he will make the kind of decisions one would expect from someone with a poor understanding of the real character of his enemy, shallow and ineffective, seemingly without reason.
That is, assuming any of this is an "attack"