The Model 940 wasn't a big seller when they were being made. If they don't sell well, the manufacturer will instead make other products that sell better. Their job is to make money, and they do this best by supplying the product the buyers most want.
The role of a back-up gun generally is to be there if the main gun breaks, is lost or taken away. To be there if needed, it's more likely there if there is room to carry it, especially carried concealed, which means a smaller, lighter gun instead of a second, full sized primary carry gun. If needed, it probably won't be fired a lot of times, but it may be fired more than the one load that it carries, and at the end of any gunfight, it is best to top off fired shots when there is time to do so safely.
In theory, a small 9mm revolver was a good back-up choice for a 9mm service pistol because it was smaller and lighter and carried the same cartridges. It could be topped off by thumbing the 9mm rounds into the revolver's chambers directly from the primary gun's spare magazine after the revolver rounds have been shot and ejected. Those rounds can still be fired, although ejecting the second, loose 5 empties would be slower than having them in moon clips.
Sure, this would be a compromise, not carrying additonal filled revolver moon clips for a reload, but all personal defense gun choices involve some compromises.
I think the 940 wasn't more of a success for several reasons, mostly that while small, it is steel framed and heavier than many of the aluminum frame .38 and .357 5 shooters the same basic size, that many of those making the choice of a back-up decided that a .38 with a speedloader or speed strips may be a more versatile pick, or perhaps one of the many small .380 pistols, or newer small 9mm pistols, with operating mechanisms more like the bigger primary arm were seen to be better choices for them. In any event, none of the several 9mm revolver models made by several makers have proven to be big sellers. With the recent exception of the popularity of .45 ACP sixguns and a few moon-clipped .40 and 10mm game guns, revolver shooters tend to like revolver cartridges and semiauto pistol shooters tend to prefer semiauto cartridges.
There are only my guesses. I have a 940 and a 547 but just about never shoot them, instead carrying and shooting a Model 642 and a .45 1911 and don't worry much about taking along 2 types of cartridges.
It's good to have choices. Even though the 940 wasn't a huge, screaming commercial success, there are enough of them out there for those who want them to be able to find one.
