Why are risk of cancer and all cause mortality dramatically reduced in vegans?

Is it because they do not consume animal products? Plausible, there are many (mostly unsung) studies showing many agents in animal products are one of the most potent carcinogens extant (see work of T. Colin Campbell for starters).

These studies are mostly unsung to this point IMHO because there is a great cultural resistance to the general assimilation of this point of view.

Or is it because fruit and vegetable consumption, independent of other factors, is extremely health promoting?

Or is it as simple as eating more cruciferous vegetables?

The debate continues, including a new variant most would consider the most extreme view, which is the so called carnivore diet where nothing is consumed but animal products, with the focus mostly on steak for most so called carnivores. There has been anecdotal successes with autoimmune conditions, most notably perhaps in the case of Mikhaila Peterson, blogger and daughter of the controversial Canadian psychologist and pop philosopher Jordan Peterson.

Obviously these anecdotals needs study. It will be interesting to see if that happens for at least two reasons: 1) it's a very charged topic from a cultural perspective, and 2) the powerful meat and dairy industries may balk at rolling the dice on this one, and they have huge political influence, which (unfortunately) holds sway over what nutritional health studies get funded.

Meanwhile I came across an interesting presentation you may find interesting on the power of cruciferous vegetables to prevent cancer, and diminish it's impact if contracted:

https://youtu.be/zz4YVJ4aRfg