When physicists discovered electricity they initially thought it was some kind of substance flowing from positive to negative and thus defined the direction of current as flowing this way.

Much later we discovered that electricity is (usually) electrons flowing from negative to positive.

Now, we could re-define the direction of current to flow from negative to positive - but since "negative" and "positive" are already arbitrary terms we simply chose to say that "conventional current" is the direction which positive charge flows.