Has Belgium "traditionally" been part of such promotions before? (honest question)
I usually only see the countries being eligible for such promotions, for which Aegean also has the corresponding language setting in the selections on the website. Also sometimes not even all of these countries would be simultaneously having an active discount code. I haven't checked all possible country codes myself during this promotion, so I don't know.

But anyway, from Brussels you certainly have quite a few options around you to start your journey from a country, which is included in the discount codes, if the savings make it worth it.