Steelix was a really good metaplay at one point, but is currently not that great. It gained a lot of popularity in a format where Luxchomp was the BDIF. With special metals, expert belt, healing cards like moomoo milk and life herb, and the Platinum blissey, steelix could overcome the quick damage that Luxchomp used to chip away at most decks. A plus was the high-hp basics as well, providing you with a deck that was largely undonkable.
There were other important considerations that again reflect the metagame at the time. Fire pokemon were hardly played (good weakness then), steelix was a stage 1 (unlike regigigas, it was good against Machamp), it had the pokebody (so unlike donphan, it was good against Crobat g), and it had resistance to psychic (good against GG and cursegar). The stadium discard was huge against gyarados as well ( not to mention other decks that used broken time-space).
Sadly, it's just not that great a card right now. Reshiram is mean to it, emboar can one shot it, and Magnezone blows it away. I wouldn't be surprised it if shows up again, but it's got a tough metagame to consider.
Hope this helped!
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