I'm not English so these are observations from afar1
. I think there are two main reasons why there aren't many good English managers:
(1) There aren't a lot of good role models for young English managers (or young English managers aren't seeking them out).
(2) The English football pyramid is sack-happy.
W/r/t (1): Think about the world's current best managers. We all have differing opinions but a generic list might include Jose Mourinho, Carlo Ancelotti, and Pep Guardiola. Now think about where these men came from. Jose Mourinho worked closely with Sir Bobby Robson at Porto and Barcelona and stayed on at Barca after Robson left to work as an assistant to Louis van Gaal. Carlo Ancelotti played for Arrigo Sacchi and served as his assistant with the Italian national team at the beginning of his career. Pep Guardiola played for and learned from Johan Cruyff, Bobby Robson, and Louis van Gaal at Barcelona; he also sought advice from and studied the tactics of Marcelo Bielsa (proving that young managers can seek out their own mentors).
Important point: the best managers of today learned from the best managers of yesterday. It's not about adhering to successful tactics of the past -- all of these men have deviated from their mentors' approaches to achieve what they have -- but they learned to think about the game from some of football's all-time greatest minds and that experience has helped them succeed over a long period of time.
Now think about where the average English prospective manager learns to think about the game. It shouldn't be too surprising that there aren't many good ones2
W/r/t (2): This is pretty apparent. Relegation is a serious fear at all levels of the English pyramid making on-the-job learning very difficult.
To revisit of generic "best managers" group -- Guardiola managed Barcelona B in the Spanish league system (a team that understandably has little relegation fear). Ancelotti started with Reggiana in Serie B. Mourinho managed Barcelona some (van Gaal let him take charge for certain games) before getting full control at Benfica, União de Leiria, and Porto3
A young manager needs time to learn from his mistakes and young English managers aren't getting it (at the Premier League and Championship level, at least, although I don't think there's much patience in League One or League Two either).
1. Although you can easily point out that we have similar problems in America (mostly (1); (2) is less problematic because nobody cares too much if you lose in the MLS).
2. It's disappointing that for all of his success in England, Sir Alex Ferguson didn't produce one capable English assistant (or maybe he did -- someone please correct me if I'm wrong). Ditto for Wenger (again, correct me if I'm wrong).
3. Yes, these guys were all instant successes. The point still stands.
Also, here's an article on this type of this thing if you're interested:http://statsbomb.com/2016/06/how-do-coaches-learn/