If you want to take things to their logical conclusion, we should be using a sort of UTC decimal time (10 hours in a day, 10 (was it 100? can't remember) minutes in an hour, 10 (same question as for minutes) seconds in a minute.
Switching to a single timezone would mean that days are upside down. Imagine some timezone where the sun rises at 23:00 and sets at 13:00. Let alone the fact that a decent chunk of the world doesn't even use 24 hour time (I'm looking especially at you, US!).