I can give you a couple reasons off the top of my head.
First, Bonneville through McNary provide a lot of flood control for the Portland/Vancouver area. The economic consequences of losing that alone would dwarf those created by removal of the lower Snake dams (which don't provide flood control on a comperable scale). For example, can you imagine all the north/south transportation corridor like I-5 and the railway cut due to flooding?
Second, the first ESA listings in the Columbia basin were of Snake river fish. The first wheel to squeek gets the grease (?).
Third, Each of the lower 4 Col. river dams generate 2-3 times more electricity than any of the lower Snake dams. Add them all together and you've got quite a difference.

There are other biology reasons that I can't think of right now ( I'm too busy wrestling the keyboard away from my two year old) but I'll get them later. Hope this helps and I'm glad this discussion is being carried on in a civil manner.
Stinkfoot