Question

Why is the semi colon called "semi" when it is used more often than the colon?

Answer 1

It refers to its use not its frequency of use.

Explanation:

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.