Most civilian Americans use the 12 hour clock with AM or PM at the end. The biggest users of the 24 hour clock would be the military, hence most Americans call it military time. Probably a lot of police and fire departments use it too.
Why, to annoy Europeans of course. Why else? Seriously, it's just a habit - we grew up with it, and have no reason to change it. Sure, 24 hours is simpler, but conversion is simple enough that it's just not an issue.
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