The fact that big pharma would lose a ton of profit is a huge factor. Also politicians are all cowards, they can't do anything radical in fear of not getting that precious re-election. So the status quo is the safest bet.
Another factor is the stoner who will spends 300 bucks on a new bong but won't donate 10 or 20 bucks to NORML or the MPP once in a while. Also is the fact that in America there is a massive amount of pressure from religious groups (separation of church and state my ass) who believe all drugs are just straight up poison (accept what big pharma pushes of course) and they are much more vocal about those opinions than the average smoker.
The biggest factor is likely that cannabis is hemp, and hemp threatens the profit margin of way more corporate conglomerates than just big pharma.
If you have not read Jack Herer's book "The Emperor Wears No Clothes" then get your ass to a book store or over to Amazon and buy it then read it twice. Seriously, what's keeping cannabis illegal is ignorance of the facts.

Smoke em if ya got em. I'm about to.
Peace all.