Know why "cherry" tastes more like "red" and "grape" tastes like "purple?"
Back in the glory days of organic chemistry they were spitting out aromatic hydrocarbons like Willy Wonka. And some of them were real, and some of 'em weren't. Vanillin - that one synthesizes easy. But benzaldehydes are only sorta related to real flavors. Nonetheless, we got all sorts of flavors that taste more like color than they taste like food.
PCP was one of a class of analgesics that got cooked up because there was a promising molecule. It had some nasty side effects. So they tried it on horses. But it wasn't as useful on horses as other horse tranquilizers. But it's easy to synthesize and its effects are highly desirable by a small class of consumers.
So it's not fair to say it was never a good idea. It just wasn't a great painkiller and let's be honest: it hasn't been used in a non-illicit environment since the mid '50s.