Because winning in Germany generally means you have to be better than Bayern. There are too many good sides in England, I'd say it's obvious. Bayern are obviously going to have some poor seasons, that's how Werder, Stuttgart and Wolfsburg won their most recent titles. In England, three or four contenders have to be poor in order for non-contender to have a chance. A little bit too much.
And let's take out Klopp for a second, I have also mentioned Allegri and Conte. What they achieved before landing two top jobs in Italy is not even close to what anyone would consider good enough for managers who could soon become part of the elite.
Top clubs are able to adress what kind of manager they need, and also to evaluate manager's potential.
I am pretty sure Pochettino will be successful after Tottenham, but regardless of that, I don't think it's fair to claim he's overrated just because of the fact he hasn't won with Tottenham.
Claudio Ranieri has won title with Leicester in a one-off season, he surely isn't better manager than Pochettino just because he made that.
Well, when English big guns had a terrible season together, with Chelsea finishing 10th, Liverpool finishing 8th, us finishing 5th and City finishing 4th, guess what, Spurs failed to win the league from the mighty Leicester City and ended up finishing 3rd in a 2 horse races. That doesn't help your first point.
You realize that no one is saying he's destined to fail with his next big club, right ? All we're saying is till he wins something or moves to a big club and win trophies, question marks will always be flying around him and rightly so. Allegri, Conte, Zidane and Pep all had question marks around them from the start, they didn't earn it easy, and they didn't get full trust from anyone. Why is Poch supposed to get full trust with showing what he has unlike these other managers ?
When he moves to a big club and has more resources, the expectations will be higher and he'll no more get excuses or a pass when he loses a trophy or even draws a game in the league. Whether he'll handle this or not will define his ability to become an elite manager, and till it happens question marks will remain around him like any manager else.