Short answer: because they were too expensive to make.

As the camera passes lovingly over all the wonderful detail, it's obvious this is not a workaday firearm. The chiseled, pierced, and engraved metalwork, the scrimshawed ivory and wire inlay, finely carved details...this was probably owned by someone with his own hunting preserve who had staff to clean his guns and prepare whatever game he shot.

But underneath all the ornament is still something quite expensive. There would be pressures of several thousand pounds per square inch at the breech when the powder charge was set off. That swinging breech block ( similar to the later Snider action) had to be tightly fitted to the receiver, and the receiver also strong enough to take that pressure. But the ( presumably steel) cartridge case would be perhaps the most critical. It would have to be a tight fit in the chamber in order to keep high-pressure gas from leaking around it and blowing open the action. Since black powder produces a lot of fouling, that tight fit could tend to become tighter and the cartridge more difficult to extract after several shots. Interestingly, the mouth of the cartridge seems to be very thin to minimize this. In theory, that could allow the front of the cartridge to expand under pressure and seal the breech, much in the way a modern brass cartridge case works.

All the fitting of those parts would have had to be done by hand. The shop would have been able to use reamers that could be expanded to scrape and enlarge the chamber, and likely the gunsmith could have used a simple turning lathe to file and fit the cartridge to the chamber. But the fitting would have been a very slow process. And, in the end, using the gun would not have been without risks. One of the merits of that upward-swinging breechblock is that if there was, say, a crack in the cartridge case and so a gas leak, the block would be blown open and the gas vented upwards- and so less likely to do very severe damage to either gun or shooter. Happily for both original owner and now auctioneer, it does not look as though that ever happened- the gun is in amazingly fine condition, considering its age.

Breechloaders of various kinds would continue to be produced for the next couple hundred years. Some of them were equally ingenious and had similar fitted cartridges ( the ones made by Jean Samuel Pauly in the early 1800's, for example) before brass cartridge ammunition became standard around the mid 19th c.. Some of those earlier designs without cartridge cases only attempted to minimize the possible dangers of high-pressure gas, like the 1819 US Hall rifle. But the Hall rifle, simple as it was in some ways, still presented enough of a challenge for mass production that quite a lot of modern machining methods were first used in the manufacturing of it at the Harper's Ferry Armory, and those methods spread to other industries. In other words, in mass producing a breechloading rifle there had to be, and was, a significant technological advance.

It's rare to be able to correct McCollum, but, first, a wheel lock does not use flint, even if it resembles a flintlock in some ways. It uses iron pyrite, and the rough wheel strikes sparks from it: unlike the flintlock, where the flint strikes sparks from the frizzen. And, second, there would be plenty of rifles and guns of this period with sights- even some crossbows had had sights.