As personal and petty as his feuds with Cabana and Hangmaam are, he sees them both as being beneath him, or at least tries to. He can look in the mirror and tell himself that he's better than the two of them, and there's enough grains of truth there that he can latch onto.
Miz main evented Wrestlemania at the period of time that Phil was white hot, the most over that he had ever been, and ever will be. Phil will never forget this, and will never forgive the Miz for this. I'm sure he could stomach Cena and the Rock the next two years, but this was for sure too bitter a pill to swallow. The Miz went somewhere that he couldn't. You just know he always saw himself as being better than the Miz, until that day. And even then, he told himself "It's fine, I'll be there next year". But next year came and went. And then the next year. And then, he was out of WWE. He'll never admit it, because it would be admitting that the Miz is better than him. He'll seethe about it to his grave.