Dialects usually develop because a population

Is isolated from authorities that would otherwise speak the standard language.

Explanation:

A dialect is a variation of a language spoken within a particular region or social group. Dialect usually develop when the standard form of the language in question is left unused or is non-present in conversations with other speakers of the same tongue.

Languages such as Spanish, French, and Italian started as dialects of Latin. The former developed as dialects due to the low-classes' lack of engagement in conversations with the high-classes, who would normally speak with a formal vocabulary and tone. The particular way of speaking among the low-classes was known as vulgar language.