Because it leaves you open and you have less power during a swing. I guess they did it for realism?
Lmao I always find it funny when someone says they did it for realism in a game like this. I mean your fighting dragons and can shoot fire out of the palms of your hands! I think realism left TES a long time ago.
It always felt natural and right and more fun to jump off some rock or cliff and slash a baddie mid air. Also we can't use weapons under water anymore either but even if we could it would be usless since just about every body of water in Skyrim seems to only be knee deep.