Copper's outer electrons are detached easily, making it great at causing an electron chain reaction. Copper is also used because of its price. While its price is increasing, its still far cheaper than other conductors.
Electricity functions by moving electrons through a circuit. As an electron on one atom is detached, it causes the electron on the atom next to it to detach as well. This chain reaction continues through the entire circuit. Copper's electrons move without much resistance, making it a great conductor.
Copper is a better conductor than gold, and its lower cost makes it the preferred option. However, there is a better conductor available: silver. While silver's high cost makes it prohibitively expensive for many tasks, its conductive performance is superior to all other metals, and it is used in certain devices.
When copper is too expensive, people often use aluminum. Aluminum performs significantly worse than copper, but its low costs makes it a better option in some scenarios. During much of the middle of the 20th century, many homeowners and builders used copper-covered aluminum wiring to save on costs. During times of war, when copper is needed for war-related tasks, electricians typically use aluminum. Aluminum is also used for large overhead electrical cables to save money and to prevent theft.