Why is steelix weak to ground?

Question

if steelix is ground/steel how is weak to ground?

Answer 1

Just because one of its type is ground doesn't mean ground wont be effective. Ground is super effective on steel types, and ground moves are neutral to other ground types. Same principle for other Pokemon, like Lucario, Terrakion, Dragon types, and Ghost types.

Answer 2

Because Steel is weak against Ground. Steelix is a Steel/Ground type but that doesn't stop the Ground type inflicting Super-Effective