Because all you're seeing is a single stint at Celtic.
He's been successful everywhere he's managed. More to the point, he's turned average teams into quality teams.
I'm not claiming he'll make us Champions, but he'll improve us significantly, of that I am quietly confident.
Not sure how much attention you want people to pay to what a manager did with Melbourne Victory or Yokohama F. Marinos.
What Postecoglu has done with Celtic is nothing more impressive than what Gerrard did with Rangers or what Rodgers did with Leicester. He's only viewed more positively than these names because he hasn't had his big job opportunity yet.
Improve us significantly from where we are now, with Mason? Probably. But he's not a better coach than Conte, or even close, so unlikely we'd reach heights of last season. We'd likely be entertaining but wildly inconsistent.