Balance has nothing to do with it, neither does people wanting to escape. Jump range has nothing to do with how good a ship is in combat, and escaping someone is so easy (jump, drop out of SC, jump) the range is irrelevant.
It's easy to understand why FD don't want a god ship that will do everything, e.g combat ships make poor traders, not great at smuggling etc so the reason for combat ships needing short jump ranges so as not to be god ships I don't buy either.
Maybe short range is designed to direct us to buy more ships because it's such a PITA to move them about? That seems a churlish game design decision to me. it's like making a motorway 1 lane just to 'persuade' drivers to take the bus... oh wait...
Also FD have said they don't want people to just own one ship, that's great and all but would it really stop us buying traders, smugglers, dedicated explorers and the like if say the FDL had a 20ly jump range or the corvette 25? I don't think so.
Another reason could be, that often increasing travel time is a useful tool for making the game environment seem larger than it is. well ED doesn't have that problem, so I don't buy that either
The real answer is, Frontier have decided that's the way it is and that's the way it will always be. It joins gender specific ranks as one of the inexplicably fixed 'we wont change' things