Laxatives are an irritant to the lining of the bowel and can cause harm if taken regularly and make the bowel lazy.
Colonics help to stimulate the action of the bowel and make the peristaltic muscle-like action stronger, whereas laxatives will make it weaker over time, making you have to use more to get your bowel moving.
Laxatives only remove some waste and can dehydrate the body whereas colonics will hydrate the body.