Alpha-Amanitin, the lethal toxin in the mushroom, doesn't play any nicer with cancer than it does with your body. So scientists are trying to tweak its wrath to focus on tumors without also annihilating your liver, kidneys, and any other squishy parts that get in its way.
The biggest issue, aside from the fact that we're talking about mainlining a substance so toxic that it's dangerous to even touch, is that mushrooms are fickle organisms. It's difficult to extract the toxin from the mushroom, so a lack of it has brought further research to a standstill. So until a synthetic strain of Alpha-Amanitin can be developed, scientists are stuck slowly wandering around under chestnut trees, far more likely to stumble across men in Birkenstocks who don't understand personal space than a life-saving deadly mushroom.