Lancelot is almost always referred to as King Arthur's (in my experience), and it is rare to even see Galahad mentioned. Knights of the Round for SNES had Arthur, Lancelot and Percival - but no Galahad. Almost anything from Hollywood with have Arthur & Lancelot as stars (Excalibur, First Knight) and Galahad is, at best, a passing character seen during a meal, or often not even mentioned. I think a large majority of people, particularly outside the medieval history / rennfest group, form their opinions based on that. In the end, Arthur dies, Lancelot gets the kingdom and the queen - and everyone loves a winner, right?
I would wager a good portion of the people who respond to the question of "Who is the most chivalrous knight of the round table?" would say "Gala-who?" if Galahad was mentioned.