Because there are no restrictions on eu players at the moment.I understand what you mean but any criteria/restrictions on foreign players will be down totally to the UK. Perhaps more complicated for EU players than now but simpler for the rest? Who knows what will emerge out of the Brexit chaos?
There are work permit restrictions for non eu players.
Therefore, and immigration was an issue with eu, post brexit it’s likely that all players will be subject to the current, non eu work permit rules.
Would be more limiting for English clubs vs eu clubs and you’d also be left with bias issues with more prominent clubs gaining favour.
Things will be different but not necessarily will everything be worse. The world wasn't going to end whatever the Brexit vote.
As stated elsewhere it's the FA who are pushing for change at the moment.