It's a super complex situation, but here is the best short version i could potentially give.
There is a big difference in between how the american sports clubs that invest into teams and european sports clubs are running their esports divisions. With NA sport franchises you very often see the pattern of buying out an already completely established franchise or getting a controlling stake. This indicates that the idea there to create a side franchise with close ties to the original franchise. This might even be due to the franchising rules of their primary sports league like NBA. I'm not that deeply invested in North American sports.
European sports clubs are usually already by default clubs that support various different sports, even though they are mostly known for their football teams. The clubs that have invested so far, with the exception of Copenhagen FC, have all build entirely new teams from the ground up, often even taking into account regionality of their players. So naturally european sports clubs move a lot slower, since they lack the already established structures that the american sports clubs simply buy out. It needs time to create something.
Then you have to take into account the big gulf in between european and north american endemic organisations as well. When an North American organisation aquires sponsors, they get paid out of the global or american marketing budget of said company. Which is a way bigger pool of money to draw from then lets say a german organisation like mousesports can draw from, since they, even though having international teams that compete on the same stage, will have to draw mostly from the marketing bugdet allocated for germany or the european union of said company.
This allows North American teams to spend way more money on salaries, especially in a game that isn't fully grown into the size that it needs to be self sufficient fincially yet. So as a player it makes way more sense trying to get onto a North American organisation or team. It is important to understand that even teams you probably never heard of are essentially working a 50+ hr week on Overwatch just for team practices alone. That doesn't even include individual practice or streaming/laddering. That is the ammount of time you need to invest in order to be competitive. Yet, the payment (if there even is one), for european teams just doesn't reflect outside of roughly 6-7 european teams. It is a financially more sound decission for an european player to play in a top 30 north american team then it is to play in a top 10 european team, unless you are in one of the big 5 teams.
And this essentially creates a self fullfilling prophecy. The teams below the big 5 have to break apart and have their players scattered all across north american teams, which then in turn makes european tournaments even less appealing because there is a draught of talent (which doesn't mean eu teams are bad at all). And with less tournaments there is even less reason to invest into european teams, so more european players will make the way across the pond.
And this isn't even factoring the OWL in. Include the OWL in the equation and then its a pretty clear decission, at least as player, in which region you should base your future in if you aren't on one of the absolute elite european teams with big sponsors and money behind them. Because even on the offchance that the OWL starts with a european division in season one, for which there is absolutely no indicator right now, it would be very likely for the majority of players who made the switch to just get recruited into the newly formed franchises and clubs that own slots if their current team doesn't make it into the NA OWL. Rationale for this is that if a org or a club is willing to spend the money that is rumored to get a slot paying the buyout on your contract would just be peanuts for them.
Hope that helps a bit <3