What more are you suppose to do with magic or a bow? You hit them with the arrow/spell... and? you're going to do what? it's not practical to give finisher moves to bows or magic, given that both can miss at the final second it is also not practical to use VAT styled cut-scenes either. VATs worked because the accuracy was decided by a % change to hit rather then a collision based hit like skyrim uses.
The above comment is beyond stupid. "% chance to hit" somehow makes finishing moves possible in programming? Really does any thinking person reason like this? "% chance to hit" does not have some magical property that somehow only a "% chance to hit" can now allow new code to include a cut scene, get real.
Finishing scene for arrows and magic are easy, the fact that those hits are collision based in no way impairs the ability to have a finishing move, to believe otherwise is to show how completely fuggen ignorant you are about programming.
One solution would be to call upon the RNG in the game with a simple subroutine against a set of defined percentages and selected casting parameters like type of spell for magic, or distance verses amount of damage for an arrow shot, then when the random percentage is meet and the other parameters achieved viola routine switches to predetermined video cut-scene just like in melee.
That is just a high level functional description but it gets the point across. The real issue was probably how much more effort was Bethesda willing to put into making the varied cut-scenes, that was more likely a bigger project than the code to allow it. There would have had to been a great deal more custom animation / artwork to support cut-scenes for arrows and magic.