Because dragons are reptiles and are therefore cold-blooded and can't survive low temperatures. Also, ice is the only thing that can pierce their otherwise inpenetrable scales.
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Dragon type Pokemon are weak against Ice, Dragon, and Fairy type moves.
Dragon types are weak to Dragon and Ice types.
Dragon and that's it, however in turn dragons are only weak against dragon and ice makeing them good types.
Druddigon is weak against dragon and ice type pokemon
ice and dragon types. glad i could help :)