Why is radiocarbon dating rarely used?

Because of the short length of the carbon-14 half-life, carbon dating is only accurate for items that are thousands to tens of thousands of years old. Most rocks of interest are much older than this.

Why do scientists use as many methods as possible when dating an artifact?

Dating techniques are procedures used by scientists to determine the age of rocks, fossils, or artifacts. Many absolute dating techniques take advantage of radioactive decay, whereby a radioactive form of an element decays into a non-radioactive product at a regular rate.