The choice is to have a city for each of the 24 hours. Then you have a module that allows easily to move the hour hand from an hour at a time (together with the 24H Day & Night indicator).
Having a half an hour indication would imply to significantly modify the module, with more complexity (hence additional size, thickness and lower legibility). I guess the question is "is it really usefull or not" and which city to choose etc...
Thus brands have to make a choice and people who need the indication for a specific timezone should set the watch to the closer city and add or remove 1/2 an hour.
Best, Mark