A per my understanding,
PDCCH is common for all user, say example you send PHICH in PDCCH then all UE will decode those data and try to match but only those UE who transmits data have to receive PHICH. One more is PHICH data is o bit data so BPSK codes but PDCCH is QPSK coded. PHICH uses grouping technique to identify the user if you include this into PDCCH then decoding PDCCH will become complicated. so to avoid all these negative impact they introduced PHICH channel.