These studies aren't randomized interventions, but I think it's a bit of a simplification to say that all have failed.
"Dietary and lifestyle determinants of mortality among German vegetarians" found that "A longer duration of vegetarianism (> or = 20 years) was associated with a lower risk, pointing to a real protective effect of this lifestyle"
"Vegetables, fruit, and cancer prevention: a review" states that "the evidence for a protective effect of greater vegetable and fruit consumption is consistent for cancers of the stomach, esophagus, lung, oral cavity and pharynx, endometrium, pancreas, and colon. The types of vegetables or fruit that most often appear to be protective against cancer are raw vegetables, followed by allium vegetables, carrots, green vegetables, cruciferous vegetables, and tomatoes"
"Nutrients and food groups and large bowel cancer in Europe" found that "most vegetables, including pulses, were inversely associated with cancer of the colon and rectum."
"Dietary and lifestyle determinants of mortality among German vegetarians" found that "A longer duration of vegetarianism (> or = 20 years) was associated with a lower risk, pointing to a real protective effect of this lifestyle"
"Vegetables, fruit, and cancer prevention: a review" states that "the evidence for a protective effect of greater vegetable and fruit consumption is consistent for cancers of the stomach, esophagus, lung, oral cavity and pharynx, endometrium, pancreas, and colon. The types of vegetables or fruit that most often appear to be protective against cancer are raw vegetables, followed by allium vegetables, carrots, green vegetables, cruciferous vegetables, and tomatoes"
"Nutrients and food groups and large bowel cancer in Europe" found that "most vegetables, including pulses, were inversely associated with cancer of the colon and rectum."