Why does a remote of one company not work for a T.V. set of another company?

Answer 1

Basically, the remote controls put out a series of short pulses of infrared light. The series of pulses is encoded so that the TV, VCR, DVD's receive circuitry can figure out what button was pressed.

Each manufacture uses their own encoding system. That is why in a Universal remote you need to put in the "code" for that manufactures device.