What if a north-south temperature gradient -- Alberta seems to have been noticeably cooler than Wyoming in Hell Creek times -- played a role?
There were many lambeosaurs and saurolophines in Alberta in the Campanian and early Maastrichtian, and both forms radiated across the Bering area in that period.
Hadrosaurs were quite big and had to come to the rivers for water, especially in times of drought, which caused mass mortality.
Drought in the Hell Creek?
Consider the variety of hadrosaurs preserved in Tsagayan and NA Campanian units. Contrast that with younger Nemegtian and Lancian beds. Hadrosaur diversity appears to have declined steadily from the late Campanian to the latest Maastrichtian.