Those studies were done in the eighties, when there first was concern over the health effects of DEHP and people started thinking they could maybe remove them from the plastic and use other plasticizers.
There were some very elegant studies done. They put the blood in glass bottles instead of bags, and they didn't have DEHP. They found the cells didn't last very well and that if they added DEHP they did last. The summary of those studies was that before we consider removing this from the bags we'd better have an alternative that will preserve the red cells.
That being said, it's not that well understood as to why that's the case.