If I recall correctly, people liked the taste of Pepsi in small amounts, but preferred Coca-Cola as a 'proper' beverage. Don't have a source at hand, so by all means correct me if I'm wrong.
This was my understanding as well, Pepsi had a sweeter taste with a slight citrus flavor. Comparatively, it tastes better on first sip, but for a can sized serving, the less sweet taste was preferred.