Because there isn't. Here's an article mentioning a research biologist who reviewed the available literature and found no evidence supporting any causative linkage:
No Link Between Pot Smoking, Cancer
Marijuana smoke contains THC, which may inhibit cancer, an expert says
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WEDNESDAY, Oct. 19 (HealthDay News) -- Even though they're chemically similar, marijuana smoke is less likely than tobacco smoke to cause cancer, according to one expert review of the literature.
The review, by Dr. Robert Melamede of the University of Colorado, Colorado Springs, noted that tobacco and marijuana smoke differ in a number of ways, particularly in the fact that marijuana smoke contains tetrahydrocannabinol (THC) , while tobacco smoke contains nicotine. Nicotine increases the cancer-promoting effects of smoke, while THC reduces those effects, he explained.
And even though THC and nicotine act on related cellular pathways, they bind to different receptors to activate these pathways, the review found. Cells in the lungs and respiratory passages are lined with nicotine receptors but these cells don't appear to have THC receptors. This may explain why smoking marijuana has so far not been linked with lung cancer, a major cause of death from cigarette smoking.
Research has also shown that marijuana kills cancer cells and reduces tumor growth. This is, in part, because marijuana reduces the formation of blood vessels that nourish tumors.
More:
http://www.healthfinder.gov/news/newsstory.asp?docID=528530 
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