I have been asking myself that question for over 50 years.
Contrary to popular belief, there has never been a time when snow geese were stupid. I started hunting them in 1969 and was bewildered by how so many of them would just fly over and never look at decoys.
Their main defense is their eyesight. A snow goose's eyes are 35 times better than yours. That means they see at 35 feet what you see at one. They also can see into the ultra-violet range, and different objects absorb or reflect ultra-violet light in different degrees. So how can you fool them?
I believe that juvenile birds' eyesight does not develop fully in the first year. November and December hunting is much easier. The juvys just do not recognize decoys as decoys, and as a result are much easier to fool. Once you can get a few juvys to commit, it is just a waiting game to get enough in the trap to pull the trigger.
The other scenarios for decoying snow geese involve different conditions that interfere with their eyesight. My favorite is fog. Hunting snow geese in the fog is one of the most exciting things...ever. To be able to hear them, but not see them, is just thrilling. I love hearing the crescendo of the approaching geese and then the sudden, exhilarating moment when they break through the fog and show themselves. It is heart stopping.
The next condition is high wind. Wind makes geese fly low and limits their range of visibility. It also makes it hard to fly, and they get tired. A tired goose is a decoy-able goose.
The last condition, and one of the least thought of by most goose hunters, is a bright sun in the gooses' eyes. Any scenario where the bright sun is at your back and in their eyes is good for you and bad for a goose.
You might wonder if any of this is rooted in fact, or just based on my personal experience hunting these birds the last 49 years. To be perfectly honest, I have been hunting them so long I cannot tell you the difference between the scientific knowledge gained by my Wildlife & Fisheries degree at Texas A&M University, work with the Texas Parks and Wildlife for years, or if this really is hands-on knowledge based on my experience. Maybe that's why they call me The Legend. I smirk a little when people call me that. Maybe there's something to it. Whatever it is though, smarts, science or Legend status, the birds aren't as stupid as we might think. That...my friends is the biggest reason they continue to be so fun to hunt.