We also don't hear about the centerfire shooters (slug shotguns, pistols and rifles) complaining because we are allowed (at least here in Ohio) to use muzzleloaders during the deer gun season...
As stated before, it all comes down to $$$...
Look at T/C Arms as an example, they made and make traditional styled muzzleloaders, now they also make in-lines...
If they (T/C) were dedicated to the traditional muzzleloaders stlye only, then the Omega & Black Diamond would have never lined their pockets with cash...
Bottom line, if We the People do not buy in-lines, then there is no demand for them and factories will eventually be forced to stop making them...