I think it starts with the Super League in general. They tend to favor starting foreign/older players. The youth development of the league is the main reason why the national team does not excel. The national team is not the place for development - they should already be developed by the time they get there.
Portugal has access to Brazilian players. Belgium and Croatia are better examples of countries we should try to emulate in terms of youth development. A few years back, Belgium did a study and found in order to be exceptional at something, in this case football, you need to devote x number of hours a week to your craft. They installed that model throughout their youth development programs and what you got is their "golden" generation. They might not have won any tourneys, but they have produced some exceptional players.