I think that there are lots of reason why there aren't more "active" or new ML shooters:
A new shooter may feel a bit overwhelmed by the amount of accessories needed to go along with their new gun.
Many recreational shooters who are pressed for time would like to go out to their local range for an hour or two and do some shooting and move on to their family or personal business. But for several reasons, muzzle loading often requires more equipment and set up time before one can even get started shooting. The pace of shooting is much slower since we are only shooting what are essentially single shot guns (except for revolvers). Then, one has to look forward to the required amount of cleaning time, when people would rather attend to other matters.
So maybe many people make a decision that they aren't going to shoot their muzzle loader as often and choose to rather shoot their more modern guns that require less time to both set up for at the range and maintenance time later.
Also, muzzle loading is not something that is accepted as an indoor range activity.
Many shooters often have a wide variety of shooting interests, both formal and informal. Because many of each of these other types of shooting activities usually require less of a committment of maintenance, equipment, effort and time, many people may believe that actively pursuing muzzle loading as a regular hobby is just not as desirable as a hobby.
Many people like to shoot their rifles accurately at longer ranges over 100 yards or more and I think many shooters can get disappointed by how consistently accurate (or inaccurate) their muzzle loading carbines perform at longer ranges.
Also, the ergonomics of the stock designs of many classical styled muzzle loaders, and heavier barrel weights often make for a less comfortable gun to hold and shoot, so this may be another factor affecting overall popularity of muzzle loading in general vs. the investment satisfaction when made in a modern centerfire rifle.
Lastly, many people may feel that any black powder shooting is simply a dirtier and messier hobby than modern guns are and is just not worth the effort. It can be dirtier on the hands, clothes, and air and also within the home during cleaning.
Now please don't think that I feel this way. Notice that I mentioned nothing about the degree of personal satisfaction one derives from muzzle loading shooting vs. modern guns, that's strictly a decision every person makes for themself. :winking: