NYSE isn't protected because it's important. NYSE is protected because it's perceived to be important; i.e., it's a high-profile terror target. Whether or not NYSE is actually important, is of secondary importance.

The same reason that public-facing corporate websites often receive more security attention than the actually-important internal network.

(e. conclusion: the recent trend of weaponizing automobiles may be a huge win for pedestrian/cyclist safety... :-\)