Here's why:
* Special Summoning Conditions: Arcana Force -0 The Fool's Special Summon condition is to have another "Arcana Force" monster on the field. There is no clause in its effect that allows it to be summoned in Defense Mode.
* Default Summoning Position: All monsters are summoned in Attack Position by default, unless specifically stated otherwise.
Therefore, when you Special Summon Arcana Force -0 The Fool, it will always be summoned in Attack Position.