The 24 hour clock option is not available. Assuming you are in the US, that is the reason. AM/PM settings are the norm here.
Also, if the clock used the 24 hour mode that would likely create a problem viewing the program guide which uses AM/PM. The guide is provided by a third party and getting that changed could be difficult.
However, I do understand your issue. Why not make a suggestion in the Verizon Idea Exchange?