Why Lakshmana could not kill Ravana?

We need to understand Ramayana in the language of Internal Ramayana taking place in the body everyday in which Rama represents consciousness, Sita the body and Lakshmana, the determined mind with an aim. 

Ravana is synonymous with ego, which the determined mind (Lakshmana) cannot kill. Only the consciousness can kill the ego. Consciousness gets activated only in parasympathetic mode, which in turn takes over only when the person has control over the breathing.

Control over breathing in mythology means your friendship with Hanumana which represents the Prana Vayu.