Steelix is part ground type, and can learn rock moves, so it can pwn that, and is unaffected by Electricity.
But I do agree that Steel should be weak to Electric, because steel has always been a conductor.
Fire, Fighting and Ground are super-effective because they can affect metal right away. Fire melts steel easily. Steel can be easily bent with brute force. Mud and sand deteriorate metals, and earthquakes bring down buildings. All of this is immediately, and isn't needed in high quantities.
Metals conduct electricity, but it DOESN'T modifies or affects metal's integrity in a physical plane. It does affect the metal in the molecular way, but it takes WAY too much electricity and a LONG exposure to it for a metal to actually resent it. Conducting isn't harmful for the metal's integrity, as it is to water's.
That's the reason. Plus, Electric is neutral against Steel for that reason. So... why not stop the complain?
As mentioned above, it's not like Electricity actually hurts Steel. it only hurts it if there's a HUGE electrical charge.
As for Water, a LOT of water will rust the metal, but that's a LOT, and for that reason, water is neutral against Steel.
I agree with the other explanations, especially Orion-Sama's. But can mud and sand deteriorate metals (and rocks, for that matter)?
My real question is, if this was about a type weakness that didn't appear to make sense... why wasn't it in the "type weaknesses that don't make sense" thread??!!