Miz is smart as shit about wrestling and how to keep the heat where the heat needs to go. Bryan and Cena are untouchable. They're both extremely good promos who can match levels with the worst Miz has to offer (or they inspire Miz to elevate his material).

On top of that, those two are completely cemented in the crowd's eyes -- Cena is what he is at this point, and Bryan is pretty well perma-over. So it's not like a feud with Ziggler where nothing he ever does sticks, and one savage line from Miz would win the crowd in his favor (for as much as Ziggler puts himself forward as the ultimate heel on social media and shit, Miz is WAY WAY better as a wrestling heel because he knows how to keep the crowd wanting to see his ass get kicked-- Ziggler was nominally a heel when he cashed in his MITB). And it's not like a feud with Strowman where Miz being a total asshole would be unbelievable, so he plays it as a chickenshit heel, which is perfect.