Here's why:
* Five-Headed Dragon is a Fusion Monster. This means it requires specific monsters to be combined on the field using a Fusion Spell card.
* Fusion Decks are built specifically to support Fusion summoning. They use Fusion Spells and monsters that facilitate combining materials.
Without a Fusion Deck, you would need to:
1. Obtain the Five-Headed Dragon card itself.
2. Obtain the Fusion Spell card required to summon it. (This is usually "Five-Headed Dragon" in its own card).
3. Have the specific monsters needed to perform the Fusion Summon on the field. (The monsters required are stated on the Five-Headed Dragon card).
It's important to note: Even if you have all these elements, you still wouldn't be able to summon Five-Headed Dragon in a non-Fusion deck because the mechanics of the game don't allow for this type of summon without the proper Fusion Spell.