Pixar films are often considered funny, light-hearted, and such, so they're perfect for the laid-backness of summer (not often, but that's the idea).
Whereas Disney always pushes for the more epic stories, family attractive stories, the "feels", which makes sense to push the films close to Christmas, so people feel more emotional about things.
Though I shan't object your former statement, Pixar's latest movies has been known for being deep, emotional tearjerkers. Besides, though Pixar can be lighthearted and fluffy, they can also make epic and grand stories as well.
And frankly, Disney haven't always had the epic quality either, though to be fair, their non-epic films are in minority. But if we're going to consider the non-epic films in the Revival era, the least epic films have been The Princess and the Frog
(though the latter wasn't a Thanksgiving release).