Answer:
Due to the presence of poor and middle class families.
Step-by-step explanation:
Brooklyn was a good place to open a birth control clinic, in Sanger's view because it was densely populated area having majority of the population belongs to middle class which had a poor economic conditions. Having more children for a middle class family brings more problems because a lot of money is needed to feed and fulfill the needs of the children so these middle class families wanted to prevent pregnancy and the only way for this prevention is the Sanger's birth control clinic.