Here's why:
* Rainbow Dark Dragon's Summoning Condition: Rainbow Dark Dragon's summoning condition is unique. It can only be special summoned by sending seven DARK monsters from your hand, field, and/or Graveyard to the Graveyard.
* Deck and Graveyard Restrictions: The effect specifically mentions "hand, field, and/or Graveyard," meaning you can't directly summon it from your deck or graveyard.
* No Special Summoning from the Graveyard: Rainbow Dark Dragon's effect doesn't involve special summoning from the graveyard. You must send monsters to the graveyard as part of its summon condition.
In Summary: While you can use monsters from your graveyard as part of the summoning condition, Rainbow Dark Dragon cannot be special summoned directly from either your deck or graveyard.