most kinds of things that fail in the body cause pain, especially if the failure is being caused by an outside agent.
Why? Well one popular just-so story is that pain exists to let us know something is wrong so we will do our best to stop the pain, if the thing that is damaging the body is some sort of external agent then it seems reasonable under this explanation that there should be pain to get you to stop that external agent in some way.
I think the more reasonable question is why did they develop a poison that was really painful?!?
I expect this is less a case of them "developing" the specific poisons as much as looking at the pharmacopeia, and choosing something that was readily available, well understood, and efficacious/unrecoverable.
maybe this is the spycraft version of designed by a developer! hmm, yes we need to make a poison that people will take to keep from being captured and tortured for years before potential release.
Ok, well I reused these components and it is cheap and easy to make! Also unimaginably painful but hey, there's always a tradeoff.
some years later, huh, research shows nobody wants to use our unimaginably painful poison to escape being tortured. Users are stupid and incomprehensible.
Why? Well one popular just-so story is that pain exists to let us know something is wrong so we will do our best to stop the pain, if the thing that is damaging the body is some sort of external agent then it seems reasonable under this explanation that there should be pain to get you to stop that external agent in some way.
I think the more reasonable question is why did they develop a poison that was really painful?!?