Esketamine doesn’t have the same activity as racemic or Arketamine (S- hits some of the dopamine receptors harder and doesn’t bind with the sigma ones iirc). Patients seem to find esketamine more pleasant because of this and it’s eliminated faster than R- so patients can (probably) get back to normal faster.

(Edit: had my right from left mixed up. It’s been a while)

Some studies suggested that the enantiomer they chose was slightly better at driving the effect people thought was causing the anti-depressive effect.

Unfortunately, newer studies suggest that 1) that effect isn't responsible for the anti-depressive effect and 2) the enantiomer they chose doesn't seem to be as good as normal ketamine.

Too soon to say, but early signs suggest this may have all been a fantastically expensive mistake. :(

They started the trials before the NIH study that proved that nmda receptor only played a small role it it's effects and that R-ket and it's metabolite was more important .

They were already too deep it

There is no rhyme or reason other than the fact that one or the other could be patented by a private co unlike Ketamine. I assume trying to patent both would've doubled their costs, so why bother, especially if it's highly probable both do the same thing.