The restoration work is not being done on the stairs themselves (which are protected by a wood covering) but of the frescoes on the walls surrounding the stairs. Over the years they became darkened and could barely be seen. Restoration will restore them to their original beauty, but has necessitated closing of the stairs themselves.
But you can still visit: Although a temporary image of the entrance has been placed over the actual entrance, an additional set of stairs has been installed next to the stairway, and pilgrims can climb those instead.
The stairs will reopen in late 2018, but we do not have a definite date at this point.
Here is a great video about the work being done from Catholic News Service.