Because they are next to impossible to animate properly in a game that has something even distantly resembling physics. That's why you very rarely see them. If you do, their animations are half-assed (but in that case, i would name the weapon "fail"). In Skyrim, you also have all the ending moves which you would have to suit for the weapon, even though i'd consider those the least of the problems when modding the weapon.
Yep, i saw some videos and they really look weird
I suppose the close we can get to a flail will be a http://www.mostinterestingfacts.com/wp-content/uploads/2010/12/3.Morning-star.jpg, no so physics dependent. But it's basically a mace so we already got em in
We don't really know how the physics will work in skyrim, tho. Afaik the engine is new. Finishing moves for a flail are not gonna be any different from the other weapons if it ignores the physics [it's more or less a scripted cut-scene], they can even script the jiggling and make it entirely physics free... but in the end it's gonna act like a "high-tech" mace of sorts.