This amazing Ted talk [1] on the potential for ketamine as a miracle anti-depressant presents at least one reason; namely, that they don't own the patents to the drugs and therefore can't monopolize their production. Instead, iterative waves of SSRIs are pushed to the public, as Big Pharma stands to profit more from SSRIs they hold the patent to, even if they're arguably much less effective and more harmful.

[1] https://www.ted.com/talks/rebecca_brachman_could_a_drug_prev...

I typed this into duckduckgo: "big pharma lobbies against drug decriminalization"

and got this: https://www.leafly.com/news/politics/the-top-5-industries-lo...

I wouldn't care to venture a guess as to their thinking but the intent is clear: they don't want drug decriminalization.