I really liked this article, which attempts to explain why gentrification is happening so quickly in Brooklyn, as opposed to other articles which are mostly about its effects.
The short version is:
Nowhere is this gap between supply and demand more apparent than in the northern Brooklyn neighborhoods of Williamsburg, Greenpoint and Bushwick. These neighborhoods are more or less frozen in 1961, when the city's zoning code restricted density and required parking in new construction. The vast majority of Williamsburg is zoned for row home-sized buildings. The city's ubiquitous six-story tenement would be illegal to build in most of Williamsburg today, as would many of the neighborhood's coveted loft buildings.
New construction sprinkled around the neighborhood stands out, but when you take into account the high ceilings, setbacks on upper floors and space that must be dedicated to parking according to the zoning code, the newest buildings almost never contain more living space than those built a century ago.
New York City's population was mostly flat or declining from 1950 onward, but surpassed its previous peak in the late 1990s due to the great employment opportunities here combined with an improved quality of life (excepting the high rents, of course). Current zoning means it is difficult to add more housing units in existing neighborhoods, and there's very little undeveloped land and similarly tight zoning in the suburbs. Thus, if housing prices in NYC are ever to stay steady or decline, rather than race upward to displace people, the city needs to allow more dense construction in existing neighborhoods, especially around the rail infrastructure that's already in place to support high density. Hopefully that happens, so people aren't forced out of their neighborhoods.