To decode the PDCCH & help in system acquisition process, UE need to know the PHICH configuration which is sent in MIB .
PHICH configuration is part of system information transmitted on PBCH & located in the first OFDM symbol of each subframe.
The physical hybrid ARQ indicator channel (PHICH) processing includes the stages of BPSK modulation,orthogonal sequence spreading, scrambling, resource element group (REG) alignment, layer mapping, precoding, PHICH group summation, and mapping unit creation.
So its better to send this all configuration to UE through first broadcast message on same set of antenna ports as the PBCH via MIB to start the system acquisition process.
I think this is the reason why PHICH sent in MIB.