Here's why:
* Summoner of Illusions' effect specifically states that the banished monster cannot be Special Summoned. This means that even though Destiny End Dragoon is in the graveyard, it's considered banished because of Summoner of Illusions' effect.
* Destiny End Dragoon's effect requires the monster to be in the graveyard, not banished. The wording of its effect is "If this card is in your Graveyard..." The effect doesn't work if the monster is banished, even if it's in the graveyard zone.
Therefore, even though Destiny End Dragoon is in the graveyard, you cannot use its effect to remove a D Hero from play and Special Summon it because it's considered banished by Summoner of Illusions' effect.