Why Belgium?

Three explosions in the airport and subway of Brussels, the capital of Belgium, left dozens of people dead today. ISIS has claimed responsibility. The attacks came four days after the capture of Salah Abdeslam near Brussels. Abdeslam is the only survivor of 10 men said to be behind the Paris terror attacks in November. ISIS also took credit for those. He was from Belgium, and he was radicalized there. The tiny country has a population of only 11 million people. But in the EU it has the highest number per capita of foreign fighters in Iraq and Syria -- more than 500 in the latest count. Why?