I don't think it's actually that rare. I know it can a problem with rainbowfish. I think because sometimes the way it presents can differ from fish to fish, people may not be able to make a correct diagnosis. Some fish can be outwardly healthy but still infected.
I think this is probably one of the more knowledgeable posts on a disease I have seen on this forum. Some of the posts on this section of the board have made me cringe with the amount of misinformation in them.
Also I don't believe in fish that TB is treatable. If it was people would not be euthanising tanks full of expensive and rare fish.