You're conflating rational and optimal. It is rational and optimal to pay off the highest rate debt first. It is rational and suboptimal to pay off the lowest debts first if you know you'll achieve success through this route. When presented with a $100k debt at 6% interest plus several $5k debts at 1-5% interest, the number of debts and the size of the large debt are overwhelming, psychologically, and people stop behaving rationally (and therefore also behave sub-optimally). The snowball method gets them to act more rationally (paying off their debts successfully), but still sub-optimally. It's closer to optimal, less debt, than the path of not paying.

EDIT: This is similar to any other sort of debt that people find themselves in. I weighed 220lbs, couldn't bench 100lbs (maybe 2 reps), and could barely run 1/4 mile at a 12-minute mile pace before wanting to pass out. So I was in a health/fitness-debt. Optimally, I could've set aside 2 hours a day to some combination of aerobic and strength training to achieve my goals in a short period of time. But that required a massive change of habits that was unlikely to stick. Instead, I ran 3 days a week (well, walk/ran at the start), learned to set aside my time for fitness activities, and improved my cardio. As I managed my time more effectively, I added in other fitness activities that got me to my strength goals as well. Adding it all at once would have (for me) been overwhelming and I likely would have failed (again) to make a routine of it. Adding in each part piece-by-piece was the far better strategy, and entirely rational, bypassing the normal anxieties of my mind by making the large change occur over several smaller changes.