What were the central causes for the genesis or creation of the tenements?

The tenements on New York's Lower East Side were described in the 1890 pioneering photojournalistic work How the Other Half Lives: Studies among the Tenements of New York by Jacob Riis.
These tenements arose in the nineteenth century. Although the main causes of their growth had to do with socioeconomic and demographic factors, it should be noted that a contributory factor was geographical. Manhattan, one of the five boroughs of New York City, is an island consisting of only 22 square miles, and even today it remains among the most densely populated areas in the world.
In the nineteenth century, immigrants flooded into Manhattan. The new wave of immigrants in this period included eastern Europeans and Italian and Irish Roman Catholics, especially the Irish who were fleeing the potato famine mid-century. These groups moved into formerly single-family houses in the Lower East Side, which had been vacated by wealthier people moving north into more fashionable districts. Real estate owners and agents made these buildings profitable by subdividing larger rooms into smaller ones so that they could rent to larger numbers of people, increasing their profits. This resulted in massive overcrowding, to which the infrastructure, especially sewage disposal, was completely inadequate. Since the new immigrants tended to be poor and unable to afford high rents, rather than maintaining buildings to make them attractive, owners invested as little as possible in the buildings to increase profits on fairly low rents.
Many of the rooms housing tenants were dark, unhealthy breeding areas for vermin, disease, and major fire hazards. Over two million people lived in tenements in the area.
https://eportfolios.macaulay.cuny.edu/tenementmuseum/history-of-tenements/