Answer 1

Because Magic Gaurd prevents damage from anything other than an enemy attack, but Explosion doesn't deal "damage" to the user, it faints the user as a direct consequence of using the move.

Comments

but I like your answer better. It's more clear. Up vote!!

Answer 2

because magic guard prevents only indirect or recoil damage.
explosion causes the user to faint 100% so it is not a recoil damage.
It is more like blowing up yourself and cause massive damage to the foe

Comments

Beat ya :P

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OMG it is so strange!!
I acatually answered first then I edited, then my answer went down!!!

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It also doesn't say something like "edited 5 seconds ago" though I edited 2 times!!

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Well, my answer didnt appear when you posted your because the page hadn't been refreshed, so you just didnt see it, and the edit thing is so you can edit things like spelling mistakes real quick wihtout it doing the "edited 30 seconds by so-and-so"

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cuz that would be so broken it would ruin the game. if this could happen, you could 6-0 teams faster than Rihana changes hairstyles....