Because the list of managers who are proven winners at the top level is very short and by no means guarantee anything:
- Pep, at City
- Heynckess, will retire
- Simeone, too defensive, also hasn't been in charge of the elite club
- Zidane, doubts about his tactics
- Ancelloti, expired
- Enrique, thanks, but no thanks
- Allegri, very good manager and a great tactician
So, people naturally look those who have the most potential to get that step up. You can't say how Tuchel, Poch or Jardim will react until you give them a chance. You can only see the work they've done at previous clubs. The mentioned category of very top managers isn't set in stone and will look different in few years time when one or two break through. To expect every manager must make a "Jose" and win a CL with "Porto" is harsh considering that the gap between the elite and those bellow is getting bigger with each passing year as the result of their growing spending power.
Personally, I really like Tuchel. Not in a sense that he should be our manager, but just that I think he has enormous potential. At this point of time, though, not just his lack of trophy, but also his personality and question marks over transfer dealings would make him highly risky appointment and I'm not sure he would handle it so well. With DoF behind him, that risk would be much lower.
Everyone logically compares him with Klopp as they both managed Dortmund and Mainz, but Tuchel is extremely interesting because, despite being not even close to Klopp's trophy cabinet, he has an ability to switch various gameplans not only before each game but during the 90 minutes. Counter-pressing, soak up the pressure or shift to possession style football when needed. That, along with innovative technics used during training makes him potentially better and more rounded than Klopp, IMO.
https://www.theguardian.com/football/2016/apr/07/thomas-tuchel-borussia-dortmund-jurgen-klopp
Football, like every other business, changes over a period of time. The ability to see, think, do things differently and creatively will always be highly valued as those people will lead the change in the end.
Of course, all this could turn out to be baloney. He never did manage a big club, has personality issues and it's a different game at the very top level. But, I can see why Bayern or other top clubs would want him.