It’s not divided into night and day like early cultures would divide time. Midday seems like the reasonable time to make the midpoint of your clock. Wikipedia says early clockfaces were 24 hours but “During the 15th and 16th centuries, the 12-hour analog dial and time system gradually became established as standard throughout Northern Europe for general public use”. Why was that?
While there is always room for more discussion and more tailored answers, this question, answered by /u/kookingpot goes into some depth regarding exactly how days were subdivided.