because the cities are supposed to be pretty long and most important it is a way to balance future actions like combat. This to give attacker and defenders a time to react. Otherwise people could log on for 10 minutes move around all over attacking without allowing others to react to it. Various "delays" in the travel system are designed to give everyone a chance to react.
Found at: http://rules.swcombine.com/general/timeframe/
“ The SW Combine is played in "delayed real time". This means each action consumes some time. When you decide to space travel, the Darkness Space Travel option will tell you how many days and hours the journey will last. These are real days and real hours. That's for the real time.
For the delay, in order to solve the problem of difference of time between the players around the world, some actions will last longer in the SW Combine (a few hours) than in real life (a few seconds). This will allow the other players to react to events happening to them while they are not connected.”