While a Pixar Land could be made that is wonderful, the problem is that the Pixar Movies do not exist in the same world (fan theories notwithstanding).
A land with rides for A Bug's Life, Toy Story, and Inside Out will leave you wondering if you're the size of a bug, toy, or human. A land with rides for WALL*E, Brave, and Up will leave you wondering if you're in the past, present, or future. Sure, you can cherry pick a few that 'go together', but you'll wind up with someone crying that their favorite isn't included because of some artificial commitment to a thematic land.
And so, a Pixar Land will have to forego a unified theme except for a big Pixar Animation sign. Now, can a really nice 'land' be made from that? Yes. Will it be Magic Kingdom 2? Oh, most definitely. Even worse. MK divides their rides into thematic lands: fantasy here, adventure here, sci-fi here. A Pixar Land will either have to re-create that approach and be exactly like MK, or, just a jumble of movie facades as you pass each IP-based ride.
Now, is there something wrong with a MK2? I don't know. There's a whole bunch of people decrying that that might be the fate of Epcot 2.0 and it makes them mad. Are they OK with a MK2 established in DHS under the banner of "Pixar Land"? We'll see...
But to answer your question, the reason why WDW probably didn't go with a Pixar Land is because they're catching up with the Harry Potter and Cars Lands. And those lands get huge praises and crowds. And so, they are committing themselves into creating areas of parks (i.e. "Lands") with full immersion dedicated to one IP, not a bunch of disparate IPs that come from one studio. Toy Story, Star Wars, and Avatar are part of that craze.