^^^^^Timo is right, I have heard it stated several times that in TOS they were trying to keep up with the concept that battles would happen at incredible speeds at incredible distances. In combat it has always been about advantages and disadvantages, so the vessel that can shoot the farthest will have an advantage of shooting a target before they can shoot you, and a ship that is faster will have the advantage of being able to move in closer before being shot at, that coupled with both ships manuevering into firing position means a lot of FTL and sub-light combat at great distances, which makes sense in a stragic point of view.
It has also been estabolished many times that they moved away from the long distance fast moving battles because fans liked to see two ships duking it out rather than a little flash of light on the viewscreen (which would be more likely).
TWOK was the first example of this, and the writers and producers specifically
made conditions that allowed for this. In the first battle the ships were close because Kirk thought it the Reliant was just in distress and not under enemy control, allowing Kahn to get as close as possible to inflict maximum damage, and the second battle they were in a nebula where sensors were limited, so they had to get close to make shots count, and at that time both ships were having engine trouble and could only go sub-light anyway.