Because when they sign contract NBA pay MORE and Euroleague was much weaker than NBA and NBA had good image.
You must live in world view not Texas view some here sounds just like Rockets fans nothing but ignorant. Let me helps you. You can ask almost any person of Europe who is Larry Bird, Michael Jordan, Magic Johnson and they will explain who they are.
Ask any person on street in Europe who is Dirk or who is Manu and they will have no idea and they will probable not know Kobe or Lebron either. But according to US fans NBA is only thing in world of basketball. This is live in past.
Ask European on street who is Papaloukas and they will know him.
Also like with Spanouli all of Houston fans says how good Houston were they actual believe can compare to Athens. Athens way of life is even superior in Mediterranean and these fans of Houston really believe such nonsense.
He give up not only money and family but also much better way of life and climate for "dream" to plays in NBA. And look what happen. And understand he is idolize the players like Jordan from this 90s era of NBA. In today NBA games is hardly even get notice on TV here but in those days was very popular.
In Greece we watch Lakers Celtics of 80s like we watch Galis. But today even basketball fans in Greece not even give damn about NBA just few like me. Maybe this help you live in bigger world to understand how Tiago maybe view things because he is not live in same box you do.