SIB is the information which is more UE- specific sent to a particular UE or a set of UEs(such as PLMN, TAC, power control etc), therefore it has to be sent over PDSCH.
MIB is the info that is more cell-specific,it is the info that UE gets first after achieving DL synch(using PSS and SSS). It contains the system bandwidth,SFN for the timing reference,antenna info,PHICH info.
Most importantly, the UE does not know the carrier (Resource Blocks) space unless or until it knows the System Bandwidth to decode the PDCCH/PDSCH. This info is in the MIB. Therefore once the UE reads it, it figure out the PDCCH/PDSCH, resource block space and all other info.