Speyguy,
I don't know for sure, but I'll give you my spin on the "why nymphing for steelhead ain't cool." Traditional fly fishing for steelhead used and still uses the wet fly swing. Even fishing dry flies, or skaters, uses the floating equivalent of the wet fly swing. In WA state, which isn't the Rockies, on fly fishing only waters, the regulations prohibited the use of weighted flies or split shot or any added weight to one's leader or fly line. (Regulations have since been modified to allow nymph fishing.) Absent any added weight, nymph fishing for steelhead simply was not practiced. Further, local fishing ettiquite used a rotation method - as in Atlantic salmon fishing - of casting and stepping downstream through the pool so that every angler gets a chance to fish the entire pool. Lead shot and upstream casting for nymphing was illegal and incompatible with the accepted fishing style.
I don't know when the regulations changed, but my guess is that WA anglers learned to nymph fish for trout from Rocky Mountain fishermen, and brought the technique home, or RM fishermen migrated to WA, take your choice, and got the technique made legal, but it's still kinda' out there on the edge of regional fly fishing morality. From what I see, nymphing seems to be Kosher for trout and not so much for steelhead. Maybe that's because 90% of a trout's diet is sub-surface. And since steelhead in freshwater are typically not feeding, we needn't bother trying to feed them nymphs.(?). Personally I don't care, and having never caught a steelhead by nymphing I fully intend to do so one day, something I've been saying for about 10 years. Since I was schooled in tradition, that's what seems most comfortable to me, and it works in most situations. But I've long thought that nymphing would allow me to fish certain types of holding water that doesn't swing.
Sg