I've asked myself before, "why stoning?" and I think I've come to an answer of some kind. Let me know if I'm off base.
There seems to be a horrifying logic in the choice of execution method and why it persists - that of diffusion of responsibility. Get everyone in the local area to chuck a few stones at the condemned, and while the overall weight of projectiles ends up killing them, each participant can then go home and sleep well at night, safe under the impression that they personally didn't land the killing blow. The stroke of a sword or the pull of a trigger is a more individual act, with the ones carrying out the act bearing more the burden of responsibility on their shoulders. If I remember the posts of
Bakunin on this kind of subject correctly, the job of executioner tends not to be a particularly long-term career choice except under specific circumstances.
Of course I reckon that this only a partial answer, but it might explain some things.