Fifty years ago there were empirical peer reviewed scientific studies that clearly indicated that bowel cancers were less common in vegetarians than in meat eaters. The word vegan was not in common usage in those days. There were however established groups of long term committed vegetarians associated with religious orders that dictated their diets. Even now there are limited studies that specifically relate to vegans. The science is comparing meat eaters to vegetarians.
A debate then arose in the 1960’s as to whether the reason vegetarians were less susceptible to bowel cancer than meat eaters was because of something in the meat, something to do with the way meat was being cooked, or possibly just because vegetarians generally ate higher fiber diets than meat eaters and the additional fiber was what was actually offering the protection.
This movement of the debate away from the obvious (that if the entire population stopped eating meat the incidents of bowel cancer would drop) was similar to what was going on in tobacco marketing at the time.
There are many peer reviewed scientific studies that have published in recent times confirming the statistical probability of getting many cancers decreases simply by not eating meat. There are also studies that clearly demonstrate the higher risk of certain cancers in meat eaters that eat processed meat.
You only need to spend a few minutes searching reputable scientific knowledge-bases you come up with a lot of information. The meat industry is a multi-billion dollar industry (like the tobacco, dairy, petrochemical, and agri-chemical industries) and has unlimited budget to fund bias studies and promote misinformation yet research citing lower cancer rates in vegetarians continues to be produced by universities and government research centers.
Here are some links to just a few recent reputable studies.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3048091/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4420687/
https://www.health.harvard.edu/blog/vegetarian-diet-linked-to-lower-colon-cancer-risk-201503117785