FWIW, I played around with the numbers a bit. Raw numbers of pictures, pictures per day, and pictures per mile were all pretty similar; the top 3 were Wyoming, Montana, and Colorado. I took more pictures per day and per mile in Washington than in Virginia, but more total pictures in Virginia.
I suspect some of my "ratings" are based on how many different kinds of pictures can you take on the route. Coming to the front range in CO, we had a couple of hazy days, so the impetus wasn't there to take lots of fuzzy pictures, as it might have been going to Colorado Springs after a cold front blew through. South Park and North Park were beautiful, but the scenery from Breckenridge to Kremmling was dominated by the dead lodgepole pines -- not particularly scenic, IMHO. How many pictures of snow covered mountains can a casual photographer (and dedicated bicyclist) take?
I don't know if the Wyoming tourist board could come up with a better route for sheer variety of scenery than the TransAm. Wind River valley and mountains, Tetons, and Yellowstone. OK, if WY were doing it, they'd route you up the east and north side of Yellowstone; the southwest side was kind of dreary, except for the geothermal sites.
As I noted in my origicnal post, Glacier skewed the results for Montana. I thought the scenery was generally better north of Missoula than south, with a few exceptions. And each of the four major passes in Washington had a different character.
Perhaps it's as much variety that leads to shooting more pictures. Both variety within a state, and something different from what I routinely see, tend to make me stop and take a picture. (And, as Russ notes, steep climbs!)
Ted, one of these days I hope to see more of Utah than the Delta hub in SLC.