Small market, and sadly, its all about the money
Top break revolvers, even with modern steels just don't have the frame strength of solid frame guns. They cannot handle the high pressures of magnum rounds. So, lots of people will pass, as they can't get a .357 or .44mag for defense.
While they can take the .45 Colt or .45acp, cost would be higher than a swing out DA revolver, for no practical benefit. As a historical replica, they sell small numbers, at pretty high prices, compared to a basic swing out DA revolver.
But yes, they are soo cool!