That's easy. UC is characterized by unpredictable periods of remission and relapse. You reduce the potential target tissue from 5-6 feet to an inch or less, there is that much less opportunity for relapse. Plus, there are varying sub-types of UC that respond to treatment differently. Fr whatever reason, cuffitis that does not respond to treatment is uncommon enough to make that risk less common than the risk of incontinence due to removal of the rectal cuff.
Nothing I have read indicates that removing the rectal cuff at the initial surgery has become the better option. It is much, much easier to screw up mucosectomy and the resulting incontinence is by far worse than the risk of cuffitis.
It is all about risk vs. benefit.
Jan