> Leftist heaps don’t try to keep the tree balanced, but just try to stop it from becoming right-heavy.
> Why can’t we use the same trick to make an efficient BST?

  The leftist heap allows us to freely swap the left tree with the right tree,
  because it does not implement any ordering of elements which depend on the its
  presence on the left or right. This is not the case for BSTs: all elements
  on the left tree must be lesser than all elements on the right tree.