For me its the challenge. I am competitive to a fault so the old saying that goes something like, "10% of the anglers catch 90% of the Steelhead" drives me bananas. I can't stand the thought of being outwitted by a fish or outfished by someone else.
I first fished steelhead out of curiosity this fall. I fished HARD on the first day...like 9 hours straight before I got my first take in some fast moving riffles right before dusk. I was tired, starving, discouraged, and my back and shoulder was screaming for relief but I will never forget the exhilaration of that take. I had never felt such a powerful animal on the end of my fly rod! I had no chance of landing it.
I did not manage to land one until my fourth day out...and I landed two, both about 23 inches that looked almost identical:
By the way, I do NOT recommend trying to land one on a 5 weight rod. It obviously can be done but of the 2 or 3 fresh fish I hooked before I landed the old one in the picture, I am not sure what was going to happen if they did not throw the hook. I feared for my equipment!