If I remember correctly, it has something to do with the number of lunar cycles in the year - a number that was/is significant to many cultures.

Stuart is correct in that 12 is an important number and derives from the 12 moon cycles per year, however that is not the only reason. Prior to circa 1300 clocks had no dial to show the time, instead they chimed the hours, and were 24 hour clocks. With the introduction of the clockwork mechanism it was more wasteful on the stored energy in the spring to chime the hour after 12, they needed winding more often, so the German and French clockmakers introduced the familar 12 hour clock face. The English and Italians continued to use the 24 hour dial until around c1550 when the 12 hour dial was adopted and has been with us ever since. It's also easier to tell the time on a 12 hour dial as the minute markers are more widely spaced.

I do have a Swatch that is a 24h. Sometimes it's just a matter of look for.

Rather busy at the moment
But here is a snippet to be going on with

24 Hr Quartz movements are still avalable I believe
Here is a watch face I designed for a 24 Hr movement



I karat of diamonds chips around a mother of pearl dial

I still have some 24hr quartz movements around somewhere
amongst the 5 to 6 thousand scattered around my work room
It uses a 5 pulse quartz movement that does 5 beats a second
So it sweeps like an automatic as oposed to 1 tick every second
They were made by Epsom who I believe belongs to Seiko

Here's one with the first 24 elements.