Is this true? If so, which chemicals and why isn't it being researched further?"

Limited research has been conducted into using chemical components found in cannabis (called cannabinoids) for medicinal purposes. To date, there is no convincing evidence to show cannabinoids are effective in treating cancer in humans. 

Tetrahydrocannabinol (THC) is the main psychoactive chemical component in cannabis, and the most biologically active. Some small laboratory studies have indicated that THC can slow tumour growth in animals. However, a small clinical trial targeting THC to brain tumours in humans was unable to conclude whether it had an impact on patient survival. Research shows that smoking cannabis (marijuana) may be a risk factor in the development of lung disease and some cancers, as well as mental illness, however research into the link between marijuana use and cancer risk has been inconsistent. 

Studies show that medicinally prescribed products containing cannabinoids can provide some symptom relief to cancer patients where unrelieved or uncontrolled by conventional treatments. Cancer Council supports further research into this area, such as the current clinical trial of nabiximols (a cannabinoid based product).