You might not associate smoking and alcohol with decreased chances of colorectal cancers, but they affect all of the body’s systems.
Both habits cause ongoing damage to cells, and often can increase the chances of cellular mutations in all functions of the body. These cell mutations, where they keep growing at faster rates than the body needs, is what causes all types of cancers. Your best bet is to quit smoking entirely, and to use alcohol in limited use for social activities only, and then sparingly. The fewer cells damaged, the better!