Blame the FDA on that one.

The FDA's policy for the last couple of decades is that mixtures of mirror images will not get FDA approval unless there is a strong rationale for it.

Racemic mixture of ketamine was approved decades ago. If you want a new indication for ketamine, you will need to get approval for a single mirror image, as the FDA won't approve the old drug.

They did this because there are numerous examples of racemic mixture drugs having increased side effects from the less active mirror image, so the FDA decided no more racemic approvals.

My guess is your insurer would be happy to pay for the old version, but your doctor is heavily incentivized to use the new version as the markup on the drug is much, much higher.