It hasn't; Mexico still mints "Onzas Plata Libertad" (Freedom Silver Ounces), with some 1.2 Oz of .999 pure silver. Such coins can be purchased at any banking establishment for some 250 pesos (USD$18.66).
Silver coins ceased to be legal tender since 1931. Silver coins for commemorative purposes however, are still minted by the Mexican government; the most important would be the Plata Libertadcoin, which has 1 oz of 99.9% pure silver.
They stopped printing greenbacks and making silver into coins.
Inflation caused the silver content of coins to be worth more than their face value so the US mint stopped making silver coins for circulation.
"They" have not stopped making 20 cent coins.
The last silver certificates were dated 1957 and there are no US bills dated 1965. By 1965 the U.S. had stopped making coins from silver and discontinued the policy of redeeming silver certificates for metal.
Americas silver is running low
it is illegal in Mexico to take coins to other countries
The government stopped putting solid silver coins out in the late 60's. Any 50 cent piece past 1969 will not be silver.
In the 1960's, the US stopped making silver coins because of the value of the metal. For example, a silver dime's silver value is worth $2 and a silver quarter is $6. Silver dollars are worth $30. The U.S. was wasting a lot of money in silver coins and the silver value was rapidly increasing, so they had to stop. Another reason would be that people took the coins, melted it and sold it to a cash-for-gold type company. Also, they would ship or smuggle them to foreign countries which is illegal.
Silver and gold were the materials of the first coins produced - they have been used as such for several thousand years.
The last silver 100-peso coins were minted in 1977, so that is the approximate date when Mexico dropped off from the silver standard.
They stopped because making the coin costed too much.
Yes, there are many silver Mexican coins, however, when compared to other countries, Mexico used lower purity silver in their more modern coins and often times different coins had different cut-off dates for silver. For example, until 1918, Mexican Peso coins were made out of 90.3% silver, in 1918 that was reduced to 80% silver, in 1920 it was reduced again to 72% silver, in 1947 it was reduced yet again to 50% silver, in 1950 it was reduced to 30% silver, and in 1957 the coin was only 10% silver until 1970 when it was replaced with a copper-nickel coin containing no silver. Mexico also has many modern bullion coins struck in silver.