why do we use copper wires as connecting wires
Because it has the best properties for the job at a price that is affordable because supplies were/are plentiful. The alternatives are either more expensive, suffer corrosion or are difficult to draw into wire. Copper has excellent conductivity, does not degrade by corrosion in normal environments, is very ductile so can easily be made into wire or cable and is not too expensive.
copper has a physical property that is a very good conductor of electricity.
we also use aluminum, though it is less conductive and tends to expand a lot more with heat. It is much lighter and cheaper. Copper has lower thermal expansion, and the highest conductivity of simple elements. (With exception for helium at very low temperature) Sometimes precious metals like gold, platinum, palladium, tungsten... and also carbon, cobalt, nickel, cadmium... all of these elements can be fused together with copper or aluminum to create a variety of properties. The biggest problem with electrical transmission is loss due to inherent resistance in the conductor. Copper is the best for very long distance transmission.
For better conductivity and durability.
To use something as a connecting wire it should possess some qualities. First it should be a good conductor of electricity, that is it should have free electrons in its configuration. Secondly the substance should be malleable, that is it should be versatile enough to be stretched into wires. And lastly it should be available in abundance and should be economically feasible. As copper satisfies all the above prerequisites it is often used to make conducting wires.
