Easier to obtain authorization to work (e.g. in the Premiership you pretty much have to be a national team regular or close to it) and popularity of English as a second language would be my guesses
Yes, it's all about legal barriers. Players who have a second passport from a country that allows them to work in the EU have a lot more options. For everyone else:
England
You need a work permit. To qualify for a work permit, you need to appear in a certain percentage of non-friendly NT games over the last year or two. So basically, if you aren't an NT regular, you aren't going to get a WP unless you get in on appeal. The ease of getting a WP via appeal has waxed and waned over the years, but certainly an unproven American kid is never going to get it.
France & Spain
France and Spain have a limit on the non-EU players they can register in their squad. These slots tend to be used for imports from African or Latin America, rather than on unproven American kids. If an American player has a passport from certain African countries, they can get into France as a "domestic" player. If an American player has a passport from signatories to the Cotonou Agreement (many countries in Africa + Caribbean), they can get into Spain. But this hasn't come up often in either country.
Italy
Serie A has a quota on the number of non-EU players that can be signed from teams outside Italy in a given year. I'm not 100% sure of the details but it also makes it hard for Americans to go here.
Netherlands
There is no roster limit on non-EU players, but there is a salary floor for non-EU players. It's not that high for youth players, but at age ~20 or so it reaches a level that is quite expensive for the bottom half of the league. Even for the bigger clubs, they really want to be sure you are worth it. Dutch clubs will sign good prospects like Richie Ledezma, but they aren't going to take flyers on any random guy.
In Germany, there are no rules about foreigners. Bundesliga clubs, who on average have a vastly smaller transfer budget than EPL clubs, also have more incentive to focus on player development as a source of top talent, and revenue. German clubs understand that you can get very good prospects for basically nothing from America, because lots of our best teens aren't locked down on professional contracts. And at this point, I think it's fair to say that success begets success. With guys like Pulisic, McKennie, and Reyna doing well in the Bundesliga, it's not a one-off fluke. Sargent also holds promise, and Adams is a very good import that RB Leipzig brought in from RBNY.
There are four Americans who have been involved with big clubs in Germany in recent years in various capacities and all have specific reasons that drew them there, but it should be noted that all wound up in their roles for specific situations and weren't just plucked out of the US like an 18 year old prospect out of the Development Academy.
David Wagner (Schalke) is a German-born-and-raised dual national. He played for the USMNT in the 90s but isn't a product of the American system in any way.
Pellegrino Matarazzo (Stuttgart) is US born-and-raised, but has been in Germany for two decades. He holds two nationalities, and local German press often refers to him as "the Italian" anyway.
Steve Cherundolo (ex-Hannover, ex-Stuttgart, youth & assistant roles) got jobs because he played for Hannover for 15 years
Jesse Marsch (ex-RB Leipzig assistant, current RB Salzburg) got a job due to his Red Bull connections.
Marsch won the Austrian double today, btw.