Someone saw reason at City Hall.
At the best of times, the marathon is a mixed bag for NY residents: great and a fun thing to watch (or even do) once or twice. It boosts tourism. It also snarls traffic hopelessly, packs the sidewalks, and turns otherwise quiet neighborhoods into bedlam. Worse, it demands an increased police presence, increased sanitation, increased engineering and transit presence, and so on. Meanwhile, there are still big chunks of the city without power. My take is that when they decided to go on, the mayor's office hadn't really understood the extent of the damage.
ETA: The marathon boosts the NYC economy by $350 million a year. Understandably, that's a lot of bucks that might be used to help recovery efforts. But that's money that only goes to emergency services partially, indirectly, and over time, in the form of taxes. And what of all the businesses that can't take advantage of the marathon, because they have no power? Closing it - still the best decision.