An Eruv usually refers to citywide or neighbourhood symbolic enclosure which allows Jews carry objects on Sabbath is if was in there private domain. There are structural limitation where an Eruv can exist such as place where 600000 people pass by.
Rabbis believe that Manhattan is “Reshut Harabim” deoraisa thus it not “eligible” for an eiruv boundaries.
“Safek deorais Lechumra” (Hebrew: ספק דאורייתא לחומרא) or
Sfeika D’Orayta L’Chumra