Why did the US stop making coins out of silver?

Answer 1

It was a matter of supply and demand. When the US first started minting coins in the 1790s the government controlled both access to silver and its price. Coins were required to contain their face value in metal, minus a tiny amount to cover the cost of minting. They were minted from an alloy containing 90% silver with 10% copper added for hardness.

The Treasury was able to control silver until the early 1960s when industrial demand for the metal in electronics and Photography (no digital cameras back then) started to outstrip supply. The government was forced to deregulate the price and relinquish control of the supply. As a result, the price increased quickly and was further inflated by speculation. At one point silver sold for as much as $50 per ounce, or close to $500 at today's rates.

Because the silver in coins had been priced at $1.29/oz, the run-up meant that it was possible to go to a bank, buy (for example) $100 worth of half-dollars at face value and immediately sell them to a metal dealer for a couple of thousand dollars. The government clearly couldn't afford to "sell" silver in the form of coins for 1/20 or 1/30 of what it was worth on the open market, so by the end of 1964 the decision was made to start making dimes and quarters out of much cheaper copper-nickel.

Unfortunately there were people in the government who felt strongly that some form of silver coinage should be maintained as a show of "stability", so the Mint continued to make half-dollars out of silver albeit using debased 40% metal instead of 90%. Even these coins were worth more than face value so they too disappeared into melting pots, effectively eliminating the half dollar as a circulating coin. When the Mint finally decided to make halves out of copper-nickel, the coin was almost never seen in change and it's stayed that way ever since.