Mark,
I agree that the rocky soil at Cascades prevented deep bunkers. However, it was also designed as a resort course, so I doubt there was much impetus to create very difficult bunkering. The club management wanted an historical restoration and not a costly remodel. That's what they got.
As for Flynn not flashing sand at many other courses outside of Merion, that is not the case. He did so more often than not. Whether they were maintained that way over the years is another matter. The bunkering at the seaside courses: Indian Creek, Boca Raton North and South, Shinnecock Hills, Atlantic City CC, Kittansett (though nearly all bunkers are raised above grade due to rocky soil), Floranada, Normandy Shores and Opa Locka all had or intended to have flashed sand faces. Of inland courses, Rolling Green, Philadelphia Country, Huntingdon Valley, Mill Road Farm, Pine Valley, TCCs in Brookline and Pepper Pike, among others all had flashed sand.
On a side note, if you want to see how difficult high flashed sand can be to maintain, go to Winged Foot after a heavy rain storm and witness all the hand maintenance that is required to push all that sand back up on the facesAs you know, Flynn was a master of designing surface drainage. Toomey and Flynn construction crews were masters of blending these drainage features beautifully into the landscape. Maybe Tillinghast wasn't quite as good