The old "there's a cure for cancer but it's being SUPPRESSED" argument. This is simply a delusional conspiracy theory and, like all of those collapses on inspection. It requires that there be a cure for all of the cancers (more on that below) and that these evil companies are all collectively holding out on us to sell their chemotherapy or whatever. But that means that someone made a cure for cancer and elected never to use it, or to use it only sparingly. And then, everyone who knows about it - which must be a ton of people, if all these BigPharma execs are in on the conspiracy - to do the same. Because each person treated with the cure makes one more potential leak in the conspiracy. And even just immediate family members of all the executives and scientists adds up quickly to a potential leak pool of thousands of people. And then! What about these evil corporations? We're meant to believe they're so evil that they would collectively allow millions to suffer and die so they can make a buck but they'll play nice and not sell each other out? The first company to do so can gold plate their entire corporate headquarters, so greed in this instance has to work against concealing the cure.
Cancer is complicated. There's not one kind, and there's no cellular "cancer button" that switches on, and can be switched off. Unless that cure is thinking nanobots that zap suspicious looking cells, there won't be a cure for cancer any more than there will be a "cure" for bacteria, or viruses, or being an asshole. We've known about cancer forever, and modern science is barely beginning to scratch its surface. To posit the existence of a single cure belies ignorance of the science involved.
Why would cannabis, or any plant, be this silver bullet that cures cancer? Humans can behave weirdly, and conspiracies have happened, but a single plant that cures (not treats) a host of human illnesses beggars belief. We've already discussed how a single cure for cancer is silly and borderline impossible, but if it exists why would a plant be making it? There's no incentive for it to do so, so it's either the biggest coincidence in the universe, or proof of god (see also, the Babel fish).
There's no evidence. This should be number 1-10 but for some people lack of evidence supports their delusional world view where comic book villains cackle evilly watching people die of cancer as they suppress evidence of its cure.