What constitutes a National Shrine:
In order for a place to be called a Shrine, certain requirements must be met. There are Diocesan Shrines, National Shrines and International Shrines.
Here are the requirements for each under Canon Law:
Can. 1230: By the term shrine is understood a church or other sacred place to which numerous members of the faithful make pilgrimage for a special reason of piety, with the approval of the local ordinary.
Can. 1231: For a shrine to be called a national shrine, the conference of bishops must give its approval; for it to be called an international shrine, the approval of the Holy See is required.
Can. 1232 §1.: The local ordinary is competent to approve the statutes of a Diocesan shrine; the conference of bishops for the statutes of a national shrine; the Holy See alone for the statutes of an international shrine.