RRC connection request message is sent by an UE when there is no radio connection with eNodeB it has.
But RRC Reestablishment request message is sent when existing radio connection goes down.
That's why RRC Connection Request just needs a single identifier as an initial identity i.e. either 5 bytes long random value or S-TMSI. But in case of Re-establishment where already existing connection broke down some other type of information required like C-RNTI, PhysCellId and shortMAC-I.
C-RNTI and PhysCellId is not sufficient to identify an UE. To make sure that Re-establishment Request message is received from a valid or genuine user, UE includes shortMAC-I IE within the RRC Connection Reestablishment Request message.