As you might know, I'm not a very good player, but, its STAB Earth Power Life Orb Sheer Forces kills a hell of a lot things, on top of that you got HP Ice as well to slay dragons. Believe me, it hits like a truck like no one else does in the tier.
Now I know many people say 'lol Ice Shard', true, but Mamoswine's usage is very low.
Landorus-I is usually paired up with Scizor/Tyranitar, so that Scizor/Tyranitar can kill things which Landorus can't such as Mamoswine, Celebi, Latias etc.
Usage doesnt have anything to do with being able to counter Landorus. Doesn't matter if it isnt used much. Skarmory tanks hits from Scizor and possibly T-Tar
I know usage doesn't have to do anything with banning, this was evident when they banned Deoxys-D as well, because its usage was in the middle 30s out of the 50 OU mons.
The thing really is that its just to easy to sweep once only likes of Mamoswine are dead.
Besides, just think, you get roughly a 60% boost without anything. Sheer Force's 30% + Life Orb's 30% which makes a move 60% stronger, without any recoil damage plus you're not locked into a single move either (Unlike Band or Specs or Scarf).
So those traits really do make it semi-broken or OP in my opinion.