I never realized it was. I actually wasn't aware of PB as a kid, it was all Willow for me. As for why, well when the genre only has so many entries, they compare to what they have, totally ignoring that W is "high fantasy" and PB is comedy/adventure.

What's interesting now is now that I can recognize film traits and patterns, the snow sled run is an obvious Lucas entry into the film; dude apparently loves his sledding scenes.