<<<<There's a stronger answer:  hanging by using tree branches, or by any other means than a gallows was against state law and had been so since colonial times.  State law mandated the the local sheriff  pick an appropriate spot and erect a gallows.  State law still governs the means of public executions.

<<<Guide books and sloppy historians (sadly including "Gotham") too often assume that if something could have happened, it did happen, and the elm long had a convenient low branch parallel to the ground  (since removed) that sure looked convenient for the role.>>>