Alder is very common with the commercial salmon smokers out here. The wood is somewhat common in certain areas around here close to the coast.. But, I've also seen it at BBQ Galore as I remember.. and with the tiny amount of wood used in the CS's.. a bag'll last quite a number of smokes.
I my be wrong.. but, I tend to believe the "traditional" woods are the woods that are most common in certain areas. We don't have hickory out here but tons of good oak. BBQ joints use a lot of oak here. Mesquite in the southwest.. pecan in the south. For a while we had a coupe of folks from Oz.. and they used eucalyptus.. I tried it and it was fine.
As we all know, certain woods go with certain product better than others. I just like mild woods with fish and birds.. stronger woods with beef and some pork.. but in my book.. OAK rules.