Another possible reason is that the study itself is flawed. The studies that have looked into this are epidemiological studies which are long term questioned based studies. Not a control group vs variable studies. Basically those who eat processed meats are also more likely to have other health habits that may put them at risk of cancer.
Those who are vegan or avoid processed meats are more likely to make other healthy decisions such as avoiding alcohol, fried foods, smoking ect and that’s why they have lower rates of cancer.