Well, there are many reasons. Tobacco smoke may be a carcinogen for development of skin cancers. It is known that in experiments with rabbits and mice that applying smoke to the skin induces squamous carcinomas. Also, the association between several malignancies at sites remote from direct smoke contact such as in the bladder, pancreas or cervix may suggest that there is a systemic carcinogenic effect from tobacco smoke. That suggests that the ingredients of tobacco smoke may induce the development of cancers.
Tobacco smoke is known to contain several carcinogens.
Tobacco smoke may set off mutations of genes resulting in the development of skin cancers. It has also been suggested that smoking may diminish the immune system. Researchers have noted that tobacco smoke does suppress the immunologic function and people who are immuno-suppressed such as though undergoing kidney transplant are known to have an increased risk of squamous carcinomas. The strength of the association, the dose dependency, the experimental evidence, the biologic plausibility, and the analogy with squamous cell carcinoma of the lung, larynx, bladder and cervix suggests causality and supports the growing evidence of smoking as a risk factor for squamous cell carcinoma in general. In order to reduce the risk of cutaneous cell carcinoma, not only should sun exposure be reduced but smoking should be discouraged as well.