Why did the government request the return of all silver certificate paper money?

Answer 1

The US never withdrew or demonetized silver certificates. You may be thinking of gold certificates, which were withdrawn in 1933 when the government ended private possession of gold as part of its efforts to restart the economy during the Great Depression.

When the Treasury deregulated the price of silver in the 1960s, nearly every $1 bill and a substantial minority of $5 and $10 bills in circulation were silver certificates. It would have been impossible to replace all of those bills quickly so instead the Treasury simply stopped exchanging them for silver metal. By declaring the bills to be "non-convertible", the Treasury simultaneously prevented people from using the bills to speculate on silver-metal price swings and avoided the disruption of recalling and replacing billions of banknotes.

Silver certificates became just another type of paper money, the same as the Federal Reserve Notes and US Notes that were also in circulation at that time. They continued to circulate for several years and were gradually removed as they wore out. The same thing happened with US Notes when they were discontinued at the end of the 1960s.