Why do some doctors say marijuana can cause cancer while other doctors say no it cant? cant we get a definitive yes or no like we did with tobacco?

No: There is no evidence that marijuana causes cancer but there is also insufficient research on marijuana - primarily due to it still being classified by the U.S. Govt as Schedule 1 which limits research on it. Therefore, those who say it does cause cancer are not basing it on any research whatsoever but there haven't been long-term trials or registeries to entirely refute that either.