Why do nickels not have ridges around the perimeter?

Answer 1

"Ridges", technically called reeding, were originally added to precious-metal coins to prevent crooks from shaving them. That practice involved using a knife or other sharp tool to scrape off a tiny amount of metal from a coin's perimeter. If done carefully it wouldn't be enough to be noticed by most people in ordinary commerce, but it could add up to a lot of money if large numbers of coins were shaved. However if the edge of a coin was reeded any attempt to scrape away its metal would be obvious and merchants wouldn't accept the coins.

Because nickels (and cents, too) weren't made of precious metals like gold or silver there was no profit to be made from shaving them so reeded edges weren't needed.

Today coins aren't made of gold or silver, but many still have reeded edges. In the US and Canada reeding is used mostly out of tradition for coins that were once made of silver. In other countries that have introduced new coins, special kinds of reeding are added to make it easy for people with visual difficulties to tell the coins apart by touch.