Well as you can infer from watching the battleviewer, ship battles are fought at sub-light speeds. So here are some reasons why ships can't flee from battles:
1) Firing up your faster than light drives takes several hours. Since most of the ships hyperspace drives are some form of advanced and complicated nuclear engines, it is easy to imagine that it could takes hours or at least many minutes to start them up (since many of our current "low tech" nuclear power plants do). Obviously when enemy ships are blasting away at you, you don't have several hours to get your engines running to jump away with ftl travel...
2) Calculating ftl travel jumps is a tricky business. All of the localized gravitational disturbances (ie: all the ships flying around in combat) makes it impossible to calculate a hyperspace jump with any chance of success. So rather than a 100% chance to obliterate themselves by jumping straight into some sort of space debris, pretty much all spaceship crews would rather take their chances with getting to an escape pod as their ship blows up in combat.
3) Pick any number of creative choices you can think of for yourself. This is all (mostly) fantasy anyways, so just let the right side of your brain come up with whatever reason it wants as to why ships can't flee from space battles