A lot of it is marketing and mindset. No one is going to market sulfasalazine on television or produce a nice shiny brochure for your GI's office. I had one doctor who wanted me to stop the sulfa because "it's an ancient
medicine." He even rolled his eyes. He wanted me to take Lialda. Newer is better!
I suppose a lot of folks are gun shy of sulfa, too, due to allergies. Some 30% of patients will have an allergic reaction to it. From what I understand, though, many of those patients can start low doses to challenge and overcome the allergy, gradually rasing their doses.
Overall, I do think part of sulfasalazine's effectiveness is that it's a systemic and topical medication whereas Asacol et al are strictly topical. I wouldn't discount its anti-bacterial effect, either. And for patients with UC-associated joint pain, I'd feel a doctor is negligence if he/ she doesn't suggest it.
And it's dirt cheap.