What is it that makes these european fighters so tough opposed to other countries?

Dude, have you ever seen a Mexican fight? Those guys are tough, it all depends on the individual you can't say people from one country or continent are tougher than others. Though Mexicans usually are known to be very tough, durable opponents in Boxing it doesn't mean all of them are. The guys you named are all from different countries, and they are all really tough (except Froch, I think people overrate his toughness and chin), but as an example Andrew Golota is also from Poland and he isn't nearly as tough as Adamek.