I believe it is price and also dealer support. You don't see many Yamaha's around here because of the dealers not really pushing them. Like gild said in a stock form a Skidoo rocks out in the mtns, I've rode with guys and thats what they ride. Another issue with the Yamahas of today is the weight, they don't like the weight of a four stroke. Yamaha's also ride rougher than say a Skidoo, Polaris, or Cat, but I know which skid will last longer and take the most abuse.
Personally I'd probably also own a Skidoo, but I've seen too many times of motor issues, one of which was that Skidoo made the bearing tolerance too small on the crank and the motor locked up.
Polaris is also a well built sled, I rode a 600 a few years back, great power, great everything, but when your powervalve bolts are shearing off and the bottoms staying in the case, and all this happens when you go down the trail, no thanks.
Arctic Cat, well I rode an M6 when the powervalve broke on the rental, wasn't impressed at all. Also when I've heard of tunnels buckling, and when a dealer has a Yamaha and a Cat line, and they drop the Cat line because they are tired of all the maintence, yea no thanks. The old cat 550s, like the ones in a 96 cougar, a AWESOME motor. The new ones, I don't like the pipe right over the spark plugs, imagine having to wait for the pipe to cool before you need to change a plug no thanks.
As of right now I stick to Yamaha because I've never been left on the trail riding one of them, or seen anyone on a Yamaha get left on the trail, the 4 strokes are awesome, and I still love the old sleds. The bulletproof triples, the reliability of the 4 strokes, and the list goes on. The Yamaha motor is what sells it to me.