The issue is salaries in Portugal are VERY VERY low - they are comparable to Poland, Romania, and Greece - yet their tax burden and cost of living is comparable to Denmark.
A 25 year old Portuguese college graduate can immigrate visa free to Denmark and double-to-triple their salary almost overnight.
The same can't be said for a Dane unless they immigrate to the US, but they're in the H1B queue like everyone else so it just isn't worth the hassle unless it's a high paying white collar job.
The EEC has a massive disparity in incomes, with average monthly wages ranging from €700 to €5,000 depending on the country, and as EEC members tend to have fairly simplified immigration policies between each other, this causes a brain drain in the countries with lower wages.