Question

Suggest a reason that the time zone do not exactly follow meridians

Answer 1

The idea of time zones is to have a particular time of day have the same meaning worldwide. For example, noon will be the point in time when the sun is the highest, if you compare from exactly on one line of a longitude line that is used to determine the time zone (actual time zone lines change based on settlements and politics, so use the longitude line it is based on). 

The sun rises and sets based on the earth's rotation on its axis. Time zones divide the earth into 24 areas that will be rotated into. Latitude lines have nothing to do with the time of day or the earth's rotation--they tell you where something is north and south rather than east and west.