Plx tell me why

The following is the logic behind this:

1. Electric energy is converted to heat energy first.
2. Heating metal (filament) will start emitting electromagnetic radiation (visible light is one of them).
3. The radiation is at a spectrum not just one wavelength.
4. Most of the energies are spent on a narrow band of wavelengths.
5. The hotter the metal, the more energies will be spent on shorter wavelengths.
6. Visible light has shorter wavelength than IR.

For objects heated up at a lower (relatively speaking) temperature, most of the heat energies are converted to invisible lights with longer wavelengths such as IR. When it is heated up to a very high temperature, more energies are converted to visible lights.

This is the concept (very simple minded) of black body radiation. This is also one of the starting point of one of the most important constants in quantum machines - the Planck's constant.

For more information, please check out the following website:
http://en.wikipedia.org/wiki/Black_body_spectrum