Discuss Abbigail Stowell's answer to: Why do you think it was a good idea to have 24 hours in a day?

Early hours were not even constant! The period of time between sunrise and sunset was simply divided by twelve, so the length of an hour would depend on the time of year as well as where you were on the planet. The decision to use equal length hours is usually credited to the Greek astronomer Hipparchus.

Nobody knows for sure why we have 24 hours - of whatever length - in a day.

Here are a few theories:
Mathematics

My favourite theory is that 24 was simply a useful number. The day was divided into 24 hours for much the same reason as the old British pound was divided into 240 pennies: it made division easier.
24 can be divided easily by 2,3,4,6,8 and 12. So dividing a full day into three shifts is easy - eight hours each. On board ship there are six watches - so each is four hours long. If you want to buy someone's time for a quarter of a day (excluding the night), that's three hours of pay. Try doing that with a day made up of ten hours!

Finger Counting

A popular theory is that the Sumerians counted in base twelve rather than in the base ten we use today. This is said to have been done using the fingers.
If you look at the fingers - not the thumb - of one hand you will see that (for most people) each contains three segments. Three segments on each of four fingers gives twelve. The thumb can then be used to point to a particular segment and indicate a number.

So, twelve segments on each hand, one hand for day and the other for night - and we have a possible origin of the 24 hour day.