The black powder business has been primarily supported by both the WWII generation and the "Baby Boomers". They're the ones who grew up with Daniel Boone, Davy Crockett, and Jeriamiah Johnson.
Well, the WWII boys are in their '80's & '90's, and us "Baby Boomers" are in their '60's & pushin' 70. In many black power clubs, the average age is 55.
The financial crash of '08 certainly had an affect on the activity. The job situation & increases in monthly healthcare cuts into disposable income. The inline action rifle represents probably 90% of the black powder market, and traditional rifle gets the remaining 10%.
The North Carolina State Muzzle Loading Rifle Association has a booth of one of the largest hunting venues in the South, the Dixie Deer Classic in Raleigh, next weekend. Every year for the past 25 years, I have passed out thousands of copies of "Muzzle Blasts", magazine of the NMLRA in an effort to sign up new members. Only a handful out of the 20,000 attendees join each year.
Since they only hunt in specific seasons, and not year-round, the Association holds little interest for them to participate in club activities. More than a hundred times when I ask folks at the Dixie Deer Classic, "what kind of black powder rifle do you have, the response is an inline rifle of various makers, oh and they have an "old" Hawken rifle back in closet, that hasn't been shot in a couple of decades.
Notice that Thompson Center Arms, maker of their "Hawken", Renegade, New Englander, and Seneca/Cherokee since the late '60's has dropped their entire line of Traditional Firearms. It's no longer popular. The inline rifle is "King".