tl;dr: Pot is illegal because of the money coming in from big Pharma companies to keep it down, since they will lose money once legalization occurs. Community Anti-Drug Coalition of America (CADCA) is hypocritical in it's war against marijuana while choosing to ignore the dangers of opioids and related prescription drugs.

Also, law enforcement is lobbying hard to keep the status quo since property seizures from pot activities are a major source of income to law enforcement since the Reagan administration.

"Legalizing marijuana could, however, hurt the bottom line of drug companies that make money off drugs like Oxycontin and Vicodin."

I find the "bottom line" argument to be pretty weak. Marijuana is not going to replace pain killers like Oxycontin or Vicodin.

Not as prescribed, no, but as an alternative recreational drug is less far-fetched. What surprises me more frankly is that no large corporations appear to be gearing up to enter the legal pot market (think BT developing solar power; can't hurt to diversify...).

If that were true, that legal marijuana leads to less abuse of "harder" drugs like opioids, then the drug companies should be happy about that.

They might have been able to make more money off their drugs when people abused them, but not really anymore. It's a pretty big social issue now and companies are trending pretty lightly in this area.

RTFA.