It's just not necessary.
The treatment of UC is fairly routine: 5-ASAs and their counterparts (rectal and oral); immunodialators (Imuran/ 6-mp); monoclonal therapies like Remicade; predisone as needed; surgery.
In most cases, treatment doesn't change. There are plenty of posts here that report a colonoscopy and a GI's recommendation that treatment continue as usual. Or, reports that a GI suggests Imuran because the ASAs aren't cutting it. In that case, what's the point of a colonoscopy? Treat the symptoms.
As an imprecise analogy, cadiologists don't order a stress test, holter monitor, or echo every time a BP medication is ineffective. They prescribe a different medication.
Going back to the late 80s and early 90s to current, there are GIs who suggest that the overuse of colonsocopy is, at best, an annoyance for the patient, but the prep, sedation, and procedure do all carry risks and costs, not to mention loss of productivity.