Finally a good question. Let me try to explain.
Since Manhattan like Yerushalayim has many visitors from all over the world, Rav Moshe maintained an eruv should not be established in Manhattan as well. However, Rav Moshe agreed (HaPardes, 33rd year, vol. 9; Kuntres Tikkun Eruvin Manhattan p. 161, and Divrei Menachem, O.C. vol. 2, siman 4) that if the rabbanim in Manhattan would erect an eruv, the precedent of Yerushalayim would not pose an obstacle. How much more so would Rav Moshe have allowed an eruv that had been erected in Brooklyn, as he was not sure if Brooklyn was even comparable to Yerushalayim at all (Igros Moshe, O.C. 1:139:6, 5:28:15).
[On the contrary, the Shulchan Aruch HaRav (364:4); Bais Ephraim (O.C. 26); Bais Meir (O.C. 364:2); Mishnah Berurah (364:5), and the Achiezer (4:8) maintain that only when the area meets all the classifications of a reshus harabbim is there a requirement of delasos.]
According to Rav Moshe, in a Brooklyn neighborhood, a tzuras hapesach would suffice for the following reason:
Since the tzuras hapesachim we erected in our neighborhoods are established in an area encompassed by mechitzos, the area is classified as a reshus hayachid, and the tzuras hapesachim are sufficient and delasos are not required (ibid., 1:139 see the end of anaf 3 where Rav Moshe states clearly that if the tzuras hapesach is erected in a reshus hayachid, it is sufficient.
3) Regarding Manhattan, Rav Moshe stated that, based on additional shitos yechidaos (shitas haRashbah and Rabeinu Ephraim), even with mechitzos and delasos at the pirtzos, Manhattan would not be classified as a reshus hayachid (ibid., 1:39:5-6).
Even these shitos yachidaos would not affect an eruv in a Brooklyn neighborhood for the following reasons: