In its early usage the semicolon served as a pause twice as long as a comma, and half as long as a colon.
Eventually an adjustment in usage became identified by grammarians.
In Semicolon, Cecelia Watson explains:
The semicolon had been transformed. Before the 1800s, it had been a pause. By the early 1800s, grammarians began to describe these pauses as means to delineate clauses properly, such that punctuation served syntax, with its prosodic and musical features secondary.
Instead of a longer pause in a single expression, the semicolon because a pause that indicates there are two distinct but dependent aspects of an expression.
