Noncompliance with meds is why a lot of diseases and conditions progress. It's not limited to UC, but UC does have its share of ineffective meds that cause serious side effects or require uncomfortable administration.
People here wonder why not every doctor pushes rectal meds. If their own experience weren't enough, there are plenty of studies that show patients just don't or won't use them.
You'd think that a transplant patient would take his or her anti-rejection meds religiously. If I'm not mistaken, though, noncompliance is one of the primary reasons for organ rejection.
One might also argue that "superbugs" are in part a result of sloppy antibiotic compliance.
It's just human nature.
On the flip side, drug companies keep trying to formulate drugs that increase compliance. Lialda and other re-packagings of 5ASAs are (at least on the surface) intended to decrease the number and frequency of pills -- but they still require multiple pills, multiple times a day.
If you want a maintenance drug that's less dependent on gut transit, go with sulfasalazine -- except people complain that it's a drug with a lot of side effects and they aren't willing to desensitize themselves to it.
It's a complicated issue.