I'm really not sure on this, but the sponsors of the contest (those providing prizes) are producers of some very high quality salmon and steelhead fly tying materials. John McClain of FeathersMc.Com has everything the tier needs to dress those beautiful full dress classic and freestyle Atlantic Salmon flies, while Ronn Lucas Sr has some of the finest exhibition quality salmon and steelhead hooks around.
That may be one reason why it's a contest for salmon and steelhead flies. The other may be that both Ronn and John are dressers of very impressive salmon and steelhead flies.
Just as an aside, there are fly dressers all over the world that have no access to steelhead or salmon fishing that dress salmon and steelhead flies for the artistic qualities of the flies, the smoothness of the silk, the sweep of the toppings and tails, the texture and coloring of the married wings and so on.
If you want to take your fly tying to the next level, give these flies a try. You might surprise yourself and enjoy fly dressing at it highest art form.