The reason is that the US dollar was originally based on the Spanish coins of the time. This meant that they had a lot of 8-real coins in circulation that coul

The reason is that the US dollar was originally based on the Spanish coins of the time. This meant that they had a lot of 8-real coins in circulation that could pass for $1, and 2-real coins that passed for 25c. People got used to that model, so as "real" American coinage scaled up, it continued to be in the 25c denomination. A 20c coin was tried to great failure in the 1870s. There was a $2.50 gold coin issued for most of the era when gold coins were issued for circulation. I note the 1-5 only scheme is fairly common too: Japan and South Korea went there (with both having a 2000 denomination note as a limited run commemorative); Russia went that way for their 1998 currency reform, but more recently added a 200 and 2000.