I think there are several factors . . .
One is "simplicity". Most folks getting in to shooting BP are getting their 'feet wet". A "Hawken" style (I'm being pretty general here) represents the half-stock percussion that was prevalent in the 1840s to the 1900s - I'm including such designs as Ohio style rifles, etc. For those that want to get involved in the historical aspect of "Rendezvous", etc. - they fit.
As far as military style - they are entirely different. A lot of folks shy away from the earlier styles as they are "smoothbores" but a good smoothbore can be accurate (plenty accurate for game whether it be rabbits or birds using shot or large game such as deer). Later models, such as the Mississippi, 1855, '61, etc. shoot mines rather than round ball (although you can shoot a patched ball in them). For some, they much prefer the "civilian" rather than the "military". Another thing is that many states allow "round ball" hunting but not "minie ball" hunting although that is quickly changing with the use of REAL slugs, etc.
I think another factor is that some folks are very shy towards flintlock - which is often vastly misunderstood. The idea of using a rock and priming powder versus a percussion seems to appear more "difficult". in reality, it's just a learning process and a good flintlock that is tuned will provide fast ignition and a flintlock rifle is just as accurate as a percussion rifle.
I have built rifles over the years and used to use a custom built Hawken for target shooting and deer hunting - the thought of lugging one around now at my age makes me break out in a sweat!
If I could keep just one "front stuffer" it would be my flintlock Fusin de Chase. Yes, it's longer than a Hawken but it's a lightweight "fusel", the lock is well tuned and gives fast ignition. As a smoothbore, it shoots plenty accurate and allows the choice of either a patched round ball or shot - ideal for anything from small game, varmints or deer.
The last thing that I think comes in to play is price. Hawke styles are mass produced and although can be on the expensive side, one is much cheaper than a custom built rifle or smoothbore. For those that want a good shooter for targets, cans or game - they allow a person to participate at a fairly reasonable price. Plus for hunters, it allows them to participate in not only regular firearm seasons but muzzleloading seasons as well.
Over the years, I've seen folks get "hung up" over the fact that they don't "have' a custom rifle like someone else. Like anything in this life - it's not about how much you have, it's about being able to "play the game" and have fun doing it. Production rifles like the Hawken styles provide that opportunity.