Well, you know what: The same is true for open heart surgeries. (One of the most incredibly invasive procedures possible.)
Actually, there are studies that support diet as an effective means of treatment.
No, there aren't enough of the "right" studies that are as "high quality" as the ones supporting the efficacy of various drugs. However, "evidence" doesn't change reality, instead, it simply reflects it.
You asked about success: For me, success means no active inflammation, at all. Which was achieved through a combination of drugs and diet.
For others, success means no longer going to the bathroom twenty times a day. Having "regular" bowel movements. No obvious symptoms.
Your comment brushed off "symptoms" as something unimportant. But, for people who are really debilitated by the disease, a change in diet that eliminates the symptoms is life changing. No need for "evidence" on that.
You're right that there's no "one" diet that works for everyone, as I mentioned in my original comment. It's a learning process. However, it's also my observation that everyone with Crohn's disease gets very aware, and very specific about what they eat. (Or, they eventually face the consequences.)