Don't know what anybody thinks, but there are multiple reasons, some of which are:
- No sugar input => low insulin (insulin promotes cancer)
- No nutrition for cancer cells => no folate etc.
- Metabolic slowdown due to starvation => means everything works slower including cancer
- Promotion of autophagy which consumes damaged organelles. This one is dubious as cancer cells can do autophagy themselves to stay alive, but I suppose that due to extensive mutations in cancer there is still a chance that normal cells do it more efficiently. In healthy parts of the body this can also lead to better immune system as damaged parts of it could be replaced in the process. On the other hand, good immunity needs various vitamins and minerals most of which can't be produced or stored.
Is this why some people think fasting can help? It gives the body a chance to clean up cancer cells, since you have more time in a catabolic state?