Question

why is copper wire not used to make filaments of electric bulbs

Answer 1

The light the we see is between 400 nm and 750 nm of wavelength. In order that a metal is able to emit this kind of electromagnetic waves by black body radiation, they have to be heated up to 2000 to 2500 deg centigrade. At lower temperatures of filaments, we do not get visible light spectrum radiated from the metals.

We can choose copper, Tungsten, Nichrome or other metal. The relevant points are Melting point, boiling point, thermal conductivity, electrical resistivity, hardness to make filaments etc. Higher melting point is needed, higher thermal conductivity and higher electrical resistivity are needed to heat up faster.

Copper and Ni-chrome will melt at these temperatures. Tungsten is very very hard substance and high melting point among metals. It will work fine.

Answer 2

The melting point of cu is less than W and  W has the hieghest  melting point so it is used in bulb