#2 CM Punk's disdain for the current WWE product
Remember when CM Punk was a part of WWE Backstage for FOX? It was not unusual to see him regularly criticize and critique the current product. Punk may feel a disconnect with what he sees in WWE, and hence AEW is a far better destination.
Here's what he told Sports Illustrated about a potential return to WWE:
"You’ve got WWE, who have multiple billion-dollar television deals, and the television’s awful. I go back there, I’m just another guy. And it’s not even that—I’d be just another guy that’s doing not-good television."
Yeah, WWE doesn't seem too likely in CM Punk's case from the looks of it.
#1 The spirit of independent wrestling is far more prominent in All Elite Wrestling
CM Punk changed the WWE landscape. No, he didn't look like the prototypical top star when he first showed up to the company. He had a very unique look and attitude, but it was different from everything we'd seen thus far. And yet he rose to the very top.
WWE may have changed over the years, but AEW embodies the independent spirit in a far better way. CM Punk, who hails from the indie wrestling world, may feel more at home there.