A readied action can trigger during a fall
The rules for falling have been clarified with this:
The rule for falling assumes that a creature immediately drops the entire distance when it falls. (XGtE)
While this occurs immediately, it does not occur instantly. An action readied with a perceivable trigger that occurs during the fall can still trigger. The assumption is initiative does not continue until the fall has ended.
For some perceivable instant, the enemy falls within your reach. Per readied actions this triggers the attack.