With JRASERVER-24547 it's now possible to drag and drop subtasks on the View Issue screen. However this is not possible in the Backlog issue detail view on Scrum/Kanban boards.

The same functionality to move the subtasks up or down with a drag and drop feature should be available on the Issue Detail View – it's possible to reorder the subtasks directly on an active sprint board or Kanban board but not within the detail view. Having a drag and drop feature would allow for quicker organization of the subtasks.