Or it more likely won because it was much better than three of the other nominees and in the opinion of more of those voting, better than Wreck-it Ralph. I could have gone either way between those two, myself.
As respected analyst PeterHale once noted, Walt Disney was the innovator of his day. His vision and focus rocked the Earth with feature-length animation. Disney assembled his "Nine Old Men," animators of skill and story crafting that shared Walt's passion. Disney took the old adage of building better mousetraps and applied it to producing better cartoons; the world came a-flocking.
But when Disney died, the company no longer had the visionary figurehead to drive them, and they settled into the comfortable and formulaic. Which, from a marketing standpoint, is sound management. It also lead to Treasure Planet and to Home On The Range.
However, another visionary, John Lasseter, thought to adapt bridge and structure software for architects into character and set modeling apps for animators. From that, Luxo, Jr. was debuted, and took the entertainment industry by storm.
Lasseter followed the footsteps of Disney, and aggregated a "posse" of fervent and thorough animators and storyboard artists, and crafted films that cribbed the best devices of comedy and drama; that these films were computer-crafted works was happenstance.
Allow me to point out that Pixar has had a positive reflexive effect on the Walt Disney Company, in that Disney has recently fielded Tangled and Wreck-it Ralph, among others. The accolades that Pixar has received are rightful. Pixar films generate both audience draw and critical acclaim. And at the end of the day, it's their results that appear on the metaphorical scoreboard.