Young adults are often the ones that are feared as potential terrorists or criminals. Why would they be traveling alone without their family?
The thing is that the journey can be very expensive, often reaching more than 10 000 euros per person. Some families just cannot afford such an important amount and decide instead to send the young and fit to find some work in Europe and eventually have enough money to bring the rest of the family. Many of the so-called single men do have a family, closing the doors on these men for security reasons might also close the door for the family that stayed in Syria.