Unlike the previous SAR contract which was very prescriptive for UKSAR2G the bidders were asked to come up with solutions which met all the requirements. As cost is a big factor in any contract bidding process the Bristow solution uses a mix of airframes with less S92 (replaced with AW189's) and some of the AW189's replaced with AW139's while still achieving all the requirements.
So all down to costs savings, nothing to do with rotor downwash.