Here's why:
* Megarock Dragon's Summoning Condition: "Megarock Dragon" can only be Special Summoned by sending 2 "Rock" monsters from your hand to the Graveyard.
* Special Summoning Rules: Special Summoning is typically a main phase action, which means you can only perform it during your own main phase.
* Opponent's Turn Restrictions: During your opponent's turn, you can only perform limited actions like setting monsters, activating traps, and using effects that specifically allow you to do so during your opponent's turn.
Therefore, you cannot Special Summon "Megarock Dragon" during your opponent's turn because it is a Special Summoning action restricted to your own main phase.