Why there are two different messages RRC Reconfig and RRC Setup?

Why there are two different messages: RRC Setup and RRC Reconfig?

Why can1t we use rrcreconfiguration at RRC setup?

RRC Setup message is for RRC connection establishment between UE and eNb.

  1. UE sends RRC Connection Request to eNb
  2. eNb sends answer to UE - RRC Connection Setup

RRC Connection Reestablishment is for any configuration changes DURING active RRC session. These changes can accur some time after connection establishment (no need do any configuration changes in RRC Setup message) and also can accur many times during active RRC session (when RRC Setup is already done)
So RRC Setup and RRC Reconfiguration are messages for different purposes.