Part of the reason they aren't more popular may be that they are huge, heavy, unweildly guns compared to a standard size gun, and there really isn't any great performance advantage in that cartridge in a pistol. You can get the same, or better, performance (similar bullet weights at similar or higher velocities) in a 475 or 500 Linebaugh, 475 or 500 Maximum, or 454 Casull without the weight/size penalty of a 45-70 revolver. The 475/500 Linebaughs are exactly the same size guns as a 44 mag or 45 Colt with more power. The new 500 S&W also makes the 45-70 revolver pretty impractical. The point of a pistol is handiness, an extra heavy, extra large pistol almost negates the point of carrying a pistol, with no advantage to be gained.
The tests from the Linebaugh seminars show the heavy loaded 45 Colt, and similar rounds penetrate as much or more test medium as many heavy rifle calibers.
The 45-70 revolvers seem like more of a novelty than a practical gun.