I think it's more the background, with better target visibility. For example, at Spanish Fork you are up on the side of a mountain with the ground falling away in front of the trap houses. The houses face NW with Lake Utah in the far background. The targets are in the blue sky almost as soon as they clear the house.
As a contrast, when I shot often a Mason MI years ago, the background was trees, and the targets were harder to pick up. At Vandalia it was hangars and the occasional airplane landing.
One of the worst backgrounds I've ever seen was out west, Mesquite NV. There was tall grass immediately in front of the house, a brushy creek bottom with geese about 100 yards out, and then a red clay hillside several hundred yards out with a Motocross race going on. Trying to pick out those orange biodegradable targets against the clay hillside with the motorcycles running about was tough, and even if you could, the targets quite often didn't break. There was at least one AA 27 AA shooter I know who went home that day when his singles score was well below 90. I had two handicap scores in the low 70s that weekend, and my scores weren't all that worse than everybody else's.
Consistent background if one important factor. Another is air density. At the higher elevations of many Western clubs, the lower air density does not disrupt patterns as much as denser air. Another factor is wind. It is windy in the West, but the wind is often rather constant and does not frequently change directions. It is not difficult to set good targets in a steady wind. It is common in the East to have the wind causing very high targets on a couple of posts and then the next post, the targets become very low. Over irregular terrain, wind changes are common.
My wife and I shot four days in Arizona last week: Ben Avery, Casa Grande and Prescott. Temperatures 60's to 70's, pure blue sky overhead although not every background was blue sky. Home, Long Island, the temperatures were teens to 20's. Even putting the temperature aside, there was no comparison. No scruffy pine backgrounds as found in the East. The birds looked like frisby's and were consistent. I felt that we were cheating. To top it off, MY WIFE KAREN SHOT HER FIRST 100 STRAIGHT at Casa Grande in one of the Arizona Chain Shoots. The folks there could not have been nicer, and they ran a great little shoot. It just ain't the same.