In NYC, why are homeless shelters full, and luxury condos vacant?

Good article in The Atlantic focusses on several factors

1. Declining demand for luxury condos:

Developers bet huge on foreign plutocrats—Russian oligarchs, Chinese moguls, Saudi royalty—looking to buy second (or seventh) homes. ... But the Chinese economy slowed, while declining oil prices dampened the demand for pieds-à-terre among Russian and Middle Eastern zillionaires. It didn’t help that the Treasury Department cracked down on attempts to launder money through fancy real estate.

2. Developers are building larger, higher-cost, housing:

First, the typical new American single-family home has become surprisingly luxurious, if not quite so swank as Manhattan’s glassy spires. Newly built houses in the U.S. are among the largest in the world, and their size-per-resident has nearly doubled in the past 50 years. And the bathrooms have multiplied. In the early ’70s, 40 percent of new single-family houses had 1.5 bathrooms or fewer; today, just 4 percent do. The mansions of the ’70s would be the typical new homes of the 2020s.

 3. Restrictions on supply (restrictive zoning) slow supply responses and make it more costly to build:

Third, and most important, the most expensive housing markets, such as San Francisco and Los Angeles, haven’t built nearly enough homes for the middle class. As urban living has become too expensive for workers, many of them have either stayed away from the richest, densest cities or moved to the south and west, where land is cheaper. This is a huge loss, not only for individual workers, but also for these metros, because denser cities offer better matches between companies and workers, and thus are richer and more productive overall. Instead of growing as they grow richer, New York City, Los Angeles, and the Bay Area are all shrinking.

The result is excess demand for cheap housing, and excess supply of expensive housing.

Young adults today are one-third less likely to own a home at this point in their lives than previous generations. Among young black Americans, homeownership has fallen to its lowest rate in more than 60 years.

So why doesn't the luxury price decline to where quantity demanded = quantity supplied?

...developers have been reluctant to slash prices too suddenly or dramatically, lest the market suddenly clear and they leave millions on the table.