I was undecided on this, but in the end it came down to the fact that it's very strong and outspeeds almost the entire metagame. It isn't that hard to revenge kill but you can't switch anything into it, and it can plow through checks with ease because of its sheer power, much like specs porygon-z. The CB set has really only one safe switch in and can 2hko a lot of resists with a stab move. Without a Steelix, you're really just guessing and hoping you don't guess wrong or you lose a pokemon.
This one goes against he popular opinion, but sharpedo is really really good. It suffers from the same problem as Durant, in that you can't switch it into anything, but once in it can cause tons of damage. There are hardly any switch ins for sharpedo, and the fact that it is generally sent out late-game means that defensive cores are usually worn down by the time it's ready to go. Ferrothorn, arguably the second best check to sharpedo, has trouble staying healthy over the course of a game due to eviolite and no recovery. Poliwrath is a reliable counter, but to be honest, it really isn't a very good pokemon in this metagame, and it's about the only hard counter in the tier. After that, there really is a drop off on the list of pokemon that can handle sharpedo. Choice band entei can check it with extremespeed, but the fact that nearly all sharpedo carry protect allows the sharpedo user to scout the entei's move and then switch out to something else that can take advantage of a choice locked normal move after already killing a pokemon (since entei isn't a great switch into sharpedo).
Offensively, it is a powerhouse. A lot of stuff has been said about how powerful it is, so I'm not gonna go into that much.
reyscarface: nails unluckiest man alive
Eternal: no ghosting Das