I'd like to offer more perspective here. I am a modern fencer (epee), a historical fencer (Meyer Longsword and Fabris Rapier).

The "ergonomic" design of the modern pistol grip is something that is suitable for the modern olympic fencing game, and is completely optimized for that specific expression of the sport. Weight, as mentioned by /u/ThomasdeCourcy, is a major reason that the pistol grip is useful for the hyper-light modern fencing swords, especially those that orient for the thrust like the epee and foil. Notice that the saber is still wielded with a traditional grip, because the biomechanics of making a "flick" as opposed to a cut are completely distinct, despite what many armchair fencers would like to argue.

Using a pistol grip with a rapier, something north of 2lbs, would actually strain the wrist far more than a normal straight hilt with quillons. It places the weapon in a relationship with the wrist that facilitates engaging the true and false edges in geometrically favorable positions against the opponent's blade, something that modern fencing has more or less moved past (not a value judgement, just an observation). Rapiers were also used quite often, despite modern ideas, for cutting, and I cannot even imagine attempting to control a cut with a rapier with a modern olympic pistol grip.

Smallswords strike a balance between the weight of the rapier and the speed of the modern foil/epee, but even then, the traditional grip helps because the smallsword game is far more often done "in the presence of blade" i.e. the blades are held together in a loose bind in order to glean more information through feeling from your opponent. Again, something that modern fencing has moved past. The traditional grip also allows a greater manipulation of the sword while keeping the point in line - something that kept the smallsword useful against heavier cutting weapons like sabers and hangers. From experience, I can tell you that even a smallsword fitted with a modern epee blade can effectively parry a meaningful cut from a historically weighted cutting sword, in a manner that allows an instant riposte.

Also consider that rapiers and smallswords, apart from their usefulness in a fight were often meant to be worn in public and displayed as fashionable accessories. Can you imagine how hideous an ergonomic pistol grip would look hanging on your waist?

The pistol grip, to make a long story short, is something that was developed for the type of fencing favored by modern olympic fencing, and should be viewed as an expression of that type of fencing. Too often, I find modern olympic fencers representing their sport as if it's an obvious and natural development of scientific swordplay. It isn't. It is a highly competitive, highly codified sport that rewards certain behaviors that create an extremely narrow set of actions and rewards (again, this is not a value judgement - I find the sport fun and challenging, and I find many of its modern aspects to be helpful in learning historical fencing, as well).

With that in mind, the pistol grip is optimized for the type of wrist movements that facilitate point-play with a weapon that is lighter than any historical sword, in a manner of use that is not intended to protect the life of its wielder. It is also intended to strain the wrist of fencers who are attending tournaments in which they will be fencing at a high intensity for one or more days at a time. That type of intensity is difficult to keep up.

Historical people, and historical fencing masters, were highly intelligent people, and there were many, many bizarre and unorthodox experiments (especially in the 18th and 19th centuries) with weapons that were meant to gives it wielder an edge, but none of them had the kind of staying power that a tradition hilt with a traditional grip did.

I will try to address the "why wasn't it used earlier" part. I would argue its a mix of tradition and practicality. The pistol grip works well with the modern light blades of sports fencing, and might work with a smallsword as well... But it would not hold well on a heavier weapon (I.e. Rapier) and would prevent you from cutting with one (yes rapiers could cut pretty well, and smallswords were often sharp too).

Furthermore in those weapons you dont bend your wrist much (as you would if you want to bring your viking sword on point). This is achieved in two ways:

• with rapier you have one or two fingers above the quillons, which gives you a nice handshake type of grip

• later smallsworders took a different approach - here you have your thumb lying on the back of the grip, which helps in bringing the point while remaining strong in the parry- It was quite possible for someone with a smallsword to face an opponent with a heavier weapon

A final point would be that there was no need for it. What existed worked well enough to meet the needs and concerns of those using it.

It was first developed for L. Visconti, an Italian master fencer who suffered damage to his hands, preventing him from using a French grip properly. And people realized that it was way better ergonomically speaking, than a French grip.

As a fencer myself, I can tell you that a pistol grip offers much, much more precise movement, strength of parry, and power. However, it also is more stressful on the hand, but for competitive fencers, it is no contest.