Why does smoking tobacco give you cancer and not weed?

Are you sure smoking weed does not contribute toward cancer? My experience has been, that the people who smoke weed also tend to be users of tobacco and often other drugs too (given the opportunity). Unless you grow and smoke your own tobacco and marijuana then you have no idea what you are putting in your body. Allot of weed gets dusted with extra dosings of psychoactive street drugs to "enhance" the effect--what's in that stuff?--it can't be good for you. They enhance tobacco the same way for taste and nicotine dosage. There's just no way to tell for sure with those drugs.

cigarettes are pumped with a lot more chemicals, and marijuana can cause cancer. It caused brain cancer for Bob Marley.

You have to understand what cancer is. Cancer cells are cells with a bad mutation (there are some mutations like CCR5 Delta 32 which are good). The reproduction of these "bad" mutating cells harms these organ or general area (Very briefly explained).

In your body right now you have a few cancerous cells that will die and not reproduce for various reasons, but everyone has cancerous cells in their body.

There are a multitude of chemicals that help cancer form or reproduce in cigarettes:

Tar - a mixture of dangerous chemicals
Arsenic - used in wood preservatives
Benzene - an industrial solvent, refined from crude oil
Cadmium - used in batteries
Formaldehyde - used in mortuaries and paint manufacturing
Polonium-210 - a highly radioactive element
Chromium - used to manufacture dye, paints and alloys
1,3-Butadiene - used in rubber manufacturing
Polycyclic aromatic hydrocarbons - a group of dangerous DNA-damaging chemicals
Nitrosamines - another group of DNA-damaging chemicals
Acrolein - formerly used as a chemical weapon

Marijuana does not have those chemicals but it still has cancer causing agents as does basically everything. However the cancer causing agents are mild compared to the ones in cigarettes.

So everything causes cancer.