Why doesn't scala give scanLeft on TraversableOnce?

scanlefttrav1.scala

  /**
   * This won't work because of the "situation" with scala collections. TraversableOnce actually
   * needs like 12 methods implemented.
   */
  class ScanLeft[A, B](it: TraversableOnce[A], init: B, fn: (B, A) => B) extends TraversableOnce[B] {
    override def foreach[U](effect: B => U): Unit = { 
      var bstate = init
      effect(bstate)
      it.foreach { a: A =>
        bstate = fn(bstate, a)
        effect(bstate)
      }   
    }   
  }
  

implicit class ScanLeft[A, Repr <: TraversableOnce[A]](val repr: Repr) extends AnyVal {  
  def scanLeft[B, That](z: B)(op: (B, A) => B)(implicit bf: scala.collection.generic.CanBuildFrom[Repr, B, That]): That = {
    val b = bf(repr)
    if (repr.isTraversableAgain) b.sizeHint(repr.size)
    var acc = z
    b += acc
    for (x <- repr) {
      acc = op(acc, x)
      b += acc
    }
    b.result
  }
}