Here's why:
* "Chaos Sorcerer" is a Normal Monster. Normal Monsters can only be Summoned by their Normal Summoning conditions, which means you need to pay the monster's Summoning cost and then place it face-up on your side of the field.
* Special Summoning is a different method of summoning. Special Summoning allows you to bring monsters from your hand, deck, or other locations to the field using specific card effects.
"Chaos Sorcerer" does not have any special effects that allow it to be Special Summoned.
Therefore, to summon "Chaos Sorcerer", you must use your Normal Summon for the turn.