The answer is quite simple really,It's all down to levels.Let me explain as best I can.
Over the years roads have been re-surfaced,but the new surface has been laid on top of the old surface thereby raising the level of the road.Pavements on the other hand are not allowed to be higher than house floor levels as they would cause a great deal of dampness.As road levels are raised then kerb levels are also raised creating a kerb level that is higher than the house floor level the only solution is a central gutter.
Hope that explained it to you.
