Why is plural of piano pianos but potato potatoes?

The plural forms of piano and potato follow different rules for forming plurals in English.

The most common way to form the plural of a noun is to add an -s to the end of the word. This rule applies to piano, so the plural form is "pianos."

However, some nouns that end in -o form their plural by adding -es. This rule applies to potato, so the plural form is "potatoes."

There are other exceptions to these general rules, so it is important to consult a dictionary or grammar guide if unsure how to form the plural of a particular word.