...beginning in about 1865, the 44.40 was the most popular cartridge for rifles and that is why many cowboys had the same caliber for their pistols...
The 44 WCF (44-40) didn't happen until '73, it was the new, more powerful centerfire cartridge for the '73 Winchester. Previous Winchesters, (1866's) used the same cartridge as the Henry, the 44 rimfire, which had a 28 gr charge. Colt didn't chamber a revolver for the 44 WCF round until '78.
Smith&Wesson had cartridge revolvers several years before Colt did. They had 44 S&W American chambered guns, and 44 Russian chambered guns as well. They were top break single action guns. I think they first came out in '68 or '69. They were fairy popular while their patents were in effect, before Colt brought out there cartridge guns. Hickok was reported to own a couple or more of them.
As to popularity, we tend to forget that not everyone instantly adopted every new thing that came out, (or could afford them) or that new guns and ammo were instantly available everywhere. Tons of surplus Civil War guns were on the market for cheap, like percussion Colt revolvers, Spencers, and converted Sharps carbines, not to mention all the muzzle loading military rifles. We know that few Colt Single Action Army revolvers reached civilian hands for a couple years, the factory was busy filling military orders. There were plenty of customers in the east as well.
"Cowboys" weren't the only people in the west, tho that's who many of us tend to think of in terms of the western US frontier in the late 1800's. There were buffalo hunters, freighters, military men, trappers, town people that had stores, hotels, blacksmiths, etc, and all the usual hangers-on around any settlement or town. There were many many miners all over also, many new towns sprung up about overnight when a gold strike became known, a whole town could appear rather quickly to cater to the mining trade and those that followed. Cattle weren't the first thing happening in many parts of the west, but came along later, after the buffalo were killed off, or seriously thinned out, Indians thinned out to a degree, and there were ways to get them to market(railroads).