Why is the semi colon called "semi" when it is used more often than the colon?
It refers to its use not its frequency of use.
Both are used as ways of breaking up sentences for a variety of reasons, perhaps changing the direction or emphasis of the point being made.
A colon places greater emphasis than a semi-colon on such a break.That is why the prefix semi is used.
However today a colon is seldom used and it is true that the semi-colon is more common.