Western bit shanks were curved for various reasons. Bits with the 90* swept back shank were so horse could graze while wearing bit, not jab himself with the curb chain. The S-curve sides were usually to prevent horse grabbing the side of bit, while being used. Other shank styles were meant to be self-correcting, so horse carried his head in the desired position. Head position wanted was not ALWAYS vertical, which was fine with some folks, because a horse with head up, nose vertical, got in the way of roping cattle. Head low, nose out in front, was much easier to rope over.
As mentioned, bits were made much less expensively from plain steel which rusted. Much more recently in Western history, have the stainless steel bits gotten common. And who could forget the multitude of chrome plated steel curb bits that came with every bridle! Also greatly popular were the aluminum bits found on all areas out west in the last century. They stayed shiny with no work, were light, humane, for the horse. Of course with time, folks changed their minds on aluminum because it rubbed off on skin, hair, couldn’t be washed or brushed off the horse.
Folks in the Western States also have a Make-Do attitude, often making bits from things around the ranch, which better suited their needs instead of spending cash to purchase a bit.
There were and are, a couple different styles of horse training, bits used in getting horses finished, from various areas of the West. Each area was greatly influenced by the people who settled there, type of terrain, in how they developed their “style” of riding and training, bits chosen to use on their horses. Texas folks way back, didn’t do so much with fancy, straight shanked bits of the spade styles. Big Basin folks do use a lot of the Spanish style, steps in training to develope a finished horse, throw a big loop because of the open ground they ride. California is Spade Bit country, has quite a well finished horse style on their animals which came from the Spanish influences. All the parts of the Spade bit are considered in building that bit for balance, feel for the horse, and almost no NEED to tighten the reins for horse to respond in turns, spins, stops. Horse feels the bit, responds as trained. If you DO tighten the reins much, folks will say your horse is NOT trained, back up and do the work needed to polish him up!
Horses of the times back then, were SMALL! So were people, so horses topping out at 15H, suited most everyone for riding and driving. Bits were fitted in the common sizes, usually the 5" worked pretty well for everyone. If not, well it was the only bit available, horse adapted himself to it. Until the improved breeding programs of the US Govt. Remount program came out, the common horses were mostly mixes and mustangs caught locally. The Remount folks stationed Gov’t owned quality breeding stallions across the west to be accessible to improve local horses, for a better quality pool of horses for the Gov’t to shop from for Cavalry replacements, There was always a need for more horses in the Armed Forces. Horses they wanted fitted a couple of types, and were chosen by experienced buyers using the criteria proven to best work in the job horse would fill. These Gov’t stallions helped tighten the body types, sizes, when crossed with the local mares, provided a more uniform colt crop as time continued.
I think the Eastern folks had more breeds of horses to use for their many specialized needs. Coach horses, Draft horses, ponies of several sizes, along with many kinds of riding horses, driving horses. With such different sizes, stores available, it was easier to stock bits in numerous sizes. Perhaps the influence of rich folks with different bits, sizing to keep their horses more happy, got the more common folks thinking of sizing bits for their animals too. Eastern folks also used a lot of steel bits, which need constant attention to keep clean and free of rust. I don’t think the stainless and never-rust in English bits got to be common until the 1940s-50s for their ease of cleaning, no rust. They might have come home with WWII soldiers from Europe. People learned other things were available in bits, which created a demand for metal bits like that. Certainly the straight shank English riding bits were the same on both sides of the Atlantic if you look at old portraits. Those folks rode with contact, wanted to MAKE SURE they could stop their horse if he got stupid during a ride, so bits had shanks of length, snug fitted curb chains. Harness racing influenced everyone into thinking that a Driving horse needed to wear a ring sided jointed bit. So almost every harness sold for YEARS came with a ring sided single joint bit as standard equipment!