Note that in 1887 Edison's Direct Current transmission was still the official, and dominant, means of distributing power in the U.S. (Link: http://en.wikipedia.org/wiki/War_of_Currents.) However, the flaw in Direct Current transmission, that would eventually lead to its discontinuance, was its extreme short range, requiring large generating stations every mile or two to supply the immediate vicinity. It is easy to imagine a large number of independent power utilities in New York City in that environment.
Also, note that it is common accounting and business practice to separately incorporate every division of a start-up company, in order to better manage risk. That there were more than 30 electrical utility corporations operating in New York City does not by any means imply that they were independently owned. In fact Westinghouse owned the exclusive rights to the use of Tesla's Alternating Current distribution technology in North America, and Edison owned all the corresponding patents for Direct Current distribution.
This was a very, very different world than today, much more akin to the PC market in the
early 1980's before IBM and Microsoft entered the fray with the IBM-PC and MS-DOS.
Consolidation really only occurred after the Niagara generating station was built in 1893, and began supplying cheap hydro-electrical power, alternating current of course, from Buffalo. This rapidly undercut all the independents.
From the Wikipedia link History of Consolidated Edison:
Con Edison’s electric business also dates back to 1882, when Thomas
Edison’s Edison Illuminating Company of New York began supplying
electricity to 59 customers in a square-mile area in lower Manhattan.
After the “War of Currents”, there were more than 30 companies
generating and distributing electricity in New York City and
Westchester County. But by 1920 there were far fewer, and the New York
Edison Company (then part of Consolidated Gas) was clearly the leader.
Note also that the large number of electrical utility companies operating in NYC at that time did not (in general) give customers a wide choice. By the nature of DC technology, the range from each generating station (of this type) was only about a mile, so potential customers had to select the nearest station. More choice was potentially available to AC customers, due to the technology's greater distribution range, but you still had to choose an operating company that was already in your neighbourhood, or pay the extremely large capital cost of financing the erection of utility poles.