The tungsten metal wire is not used in the heater as heating element. why?

Tungsten metal reacts with oxygen with air at high temperature and oxidized . So tungsten metal wire is not used in a heater. 

Tungsten oxidizes rapidly in air at high temperatures. Heating elements are generally made from alloys like Nichrome or Kanthal which won't oxidize. Tungsten has a much higher melting point, but it needs an inert atmosphere or a vacuum, such as inside an incandescent lamp.due to very high melting point the tungsten wire cannot used in elctric heater and another reason is that due to heating effect of current.

Tungsten is a poor conductor of electricity even though it is a metal. But at high temperatures, it will conduct electricity. As discussed above, it has a high melting point. Therefore, high temperature doesn't have any bad effect on Tungsten.Tungsten and Bismuth are metals which are poor conductors of electricity. There are many, but some include Aluminum, Bismuth, Gallium, Indium, Lead, Thallium, Tin, Ununhexium, Ununpentium, Ununquadium, and Ununtrium.

Nichrome: Most resistance wire heating elements use nichrome 80/20 (80% Nickel, 20% Chromium) wire, ribbon, or strip. Nichrome 80/20 is an ideal material, because it has relatively high resistance and forms an adherent layer of chromium oxide when it is heated for the first time.