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.
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
How to change your username
How to add a user avatar
Why was my post hidden?
Flags vs Downvotes
Official PokemonDb League
Type a Pokemon name, type, move or ability to see instant results.