It's to do with the salience/relevance of the information: on average the month is the same for, well, half a month (yeah stats!) - so the more salient information is the more variable day - so that gets mentioned first.
Also by ordering the information by variability, it allows us to easily drop the irrelevant or assumed known backdrop info without having to re-order ourselves.
It's Wednesday 14th March 2018
It's Wednesday 14th March
It's Wednesday 14th