Why doesn't traverseU work with State? Why must traverseS exist as a special case?

traverseU vs traverseS

scala> import scalaz._, Scalaz._, Unapply._
import scalaz._
import Scalaz._
import Unapply._

scala> List(1, 2, 3).traverseU((i: Int) => i.pure[({type f[a] = State[Int, a]})#f])
<console>:17: error: Unable to unapply type `scalaz.IndexedStateT[[+X]X,Int,Int,Int]` into a type constructor of kind `M[_]` that is classified by the type class `scalaz.Applicative`
1) Check that the type class is defined by compiling `implicitly[scalaz.Applicative[<type constructor>]]`.
2) Review the implicits in object Unapply, which only cover common type 'shapes'
(implicit not found: scalaz.Unapply[scalaz.Applicative, scalaz.IndexedStateT[[+X]X,Int,Int,Int]])
              List(1, 2, 3).traverseU((i: Int) => i.pure[({type f[a] = State[Int, a]})#f])
                                     ^

scala> List(1, 2, 3).traverseS((i: Int) => i.pure[({type f[a] = State[Int, a]})#f])
res8: scalaz.State[Int,List[Int]] = scalaz.package$State$$anon$3@414e3789

I put this question on SO if you want to answer it: http://stackoverflow.com/questions/16247126/scalaz-why-doesnt-traverseu-work-with-state-why-must-traverses-exist