The first "professional" pistoleros were duelists, fighting under a very formal set of rules. They adopted a one-handed, edge-on stance for a number of reasons:
1) To present the smallest possible target (without doing something ungentlemanly, such as dropping to the ground
)
2) To put the front sight of the pistol as far from the dominant eye as possible
3) To get into shooting position as quickly as possible while turning to face the opponent
4) Because the other favorite duelling weapon, the sword, was used in a similar stance
OK, so I'm just guessing...
Also, the other early users of handguns were cavalrymen, who had to shoot one-handed while using the other hand to control their horses.
Finally, early handgun technique evolved in the age of single shot pistols. Managing recoil for a quick second second shot wasn't a consideration.