Up also went a bit bland and rote in its plotting after the house arrived in South America, regardless of the perfection of that first sequence. My point being that I'd be willing to bet that what sold the movie to the millions that went to see it was the by-the-numbers adventure part and not the sweet, beautiful, sad and wonderful part.
Incidentally, a similar thing happened with Wall-e which took a nosedive after the little guy left the planet.
Also, Ice Age, or Madagascar, or whatever made millions and millions of dollars, despite being a lot worse than everything Pixar ever made, including Cars. Popularity simply isn't the best gauge of quality, even if the two are often times related.