A key difference between the situations of Bayern or Juventus vis-à-vis United is that their primacy in the league is rarely challenged (at least in a current context, where you don't have a Berlusconi-powered Milan to consistently challenge the latter), so they can afford to be more a bit adventurous with their choices (as there's more margin for error). Bayern could hypothetically nab Jaissle from Salzburg or Hoeneß from Hoffenheim or Svensson from Mainz , and their short or medium term objectives won't suffer if the appointment goes south; but even then, they have mostly chosen to employ established managers like van Gaal, Heynckes, Guardiola, Ancelotti over the last 15 years or so — and hopeful appointments like Kovač from Eintracht Frankfurt were rather sporadic (speaking of permanent choices, not interim like Sagnol).
In the Premier League, any new United manager will have his back against the wall from the very beginning — as he will have to stare down three of the foremost tacticians of the contemporary game in Guardiola, Klopp and Tuchel in a race to the very top (which isn't happening in Serie A right now...Gasperini, Mourinho, Inzaghi, Spalletti, Pioli are at a lower level in comparison). Not to mention, the revenues of Manchester City, Liverpool and Chelsea are now within touching distance of United (whereas Bayern and Juventus have a substantial lead over the chasing pack), which makes things even more competitive. There's a tangible threat of one (or more) of them matching or surpassing United in the years to come if we keep stabbing ourselves in the foot...
Given what's at stake and the nature of the competition in the uppermost echelons of the Premier League (domestically, and in Europe: with Liverpool, City and Chelsea winning or reaching the finals of the Champions League of late), it does make sense to minimize unnecessary risks and opt for a top-of-the-line manager (depending on availability of course) rather than an aspirant. Potter is a good coach, pretty resourceful and has his team punching above its weight...with Brighton play an eye-catching brand of football under him (like Rodgers at Swansea) — should definitely be looked at as there's a lot to like about him. But, they're a middling or lower-half-of-the-table team (where the pressure is substantially lower), and he has only one season of Europa League experience under his belt from Östersund...that should give any reasonably objective person some pause.
Whereas someone like Nagelsmann (also a bright, up-and-coming manager at face value) had finished in the Top 4 with Hoffenheim on separate occasions, before guiding Leipzig to the Champions League semi-final vs. Paris Saint-Germain and finishing runner-up to Flick's Bayern — so there are a lot of fundamental differences between his resumé in terms of elite-experience and overall pedigree (before Bayern came calling) and Potter's, and a lot more question marks around the latter. For what it's worth, there are some question marks around the other candidates, too — in terms of rigorousness of domestic competition, footballing ideology, squad-building nous, man management, availability — and there are no near-perfect choices like Guardiola or Klopp lying in wait for us, so United will have to be ultra judicious and selective.
