NYC had a better port. Back when this mattered, a deep-water port at the end of the Hudson river was a big deal (D.C. was never in the running, but up until 1815 or so Boston and Philly were bigger commercial centers than NYC.) For the first 100-150 years of its existence the early colonies traded more with England than with each other, so a good port was a necessity for a city but it did not need much else beyond that. Post-revolution most trade was still external rather than internal but England and France shut down most of this trade during the Napoleonic wars. At this point the advantage went to NYC because by sitting at the end of the Hudson river it was able to serve as the trans-shipment point for goods coming down from the Hudson river valley out to the other states. After the Gibbons v. Ogden court case established federalist supremacy over interstate commerce the maritime merchants of NYC were better positioned than their competitors in Boston, Philly, or Baltimore to take advantage of this explosion of commercial opportunity.

The reason people gathered at a certain buttonwood tree in lower manhattan was to trade shipping interests -- early finance was all about commercial shipping and the fact that NYC was an early leader in shipping led to the development of a finance hub and the two sides of the same coin propped each other up.