Temporary ostomies are sometimes used in Crohn's to divert the stool from an area while it heals; a newly-created anastomosis, perianal disease, etc. So diversion does work in some cases, otherwise there would be no point in doing it.
But the colon is relatively massive and diverting the stool doesn't seem to heal it. In fact, stool diversion can create a form of inflammation in its own right known as diversion colitis, and distinct from IBD (though usually not as severe).
@Gary - The topic creator made it quite plain it wouldn't be a cure; you seem to be arguing with them over something they never even said. The concept of 'bowel rest' is not a new one; it has been used as a treatment for Crohn's for a long time. It works best for small bowel disease and it 'works' in the sense a course of steroids works (but without the toxic side-effects). But unfortunately bowel rest doesn't seem to induce even a temporary remission in severe UC.
Another link which nobody will read.
www.uptodate.com/contents/management-of-severe-ulcerative-colitis/abstract/5,6