From Big Horn's website:
Q:Will you chamber the Model 89 in .460 Smith & Wesson?
A:There are no definite plans for one at this writing. Unfortunately, we haven't solved the difficult parts concerning the .460 S&W Magnum. We have considered creating a lever gun in .460 S&W Magnum, but this cartridge has a significantly higher pressure maximum. While we believe our action could stand up to such pressure, we would not offer it until we would have a chance to do a great deal of testing.
Aside from the pressure issue, the fact that the feeding of lever guns are very sensitive to cartridge length causes us great concern were we to offer a .460. We know people would like to use it for .45 Colt and .454 Casull, just like they could in a revolver. Since these cartridges have a maximum overall length of approximately 1/2" less than the .460 S&W Magnum, we are fairly certain that the shorter cartridges would not feed properly through an action set up for the .460 S&W. We don't want to sell a product that would cause our customers to be unhappy.
Maybe the reason it hasn't been done by other manufacturers.