Because every class cant see a sniper. Can a sniper be spot and marked, can you defuse a OHK bullet once it hits you? This is why it is acceptable for it to be a OHK weapon. There are so many downsides to using them with everything in place not to mention it is based on chance someone actually steps on it and keeps moving instead of waiting for a teammate to come defuse.
Grenades shouldn't be a OHK because everyone has grenades and they can be spammed by a team. Can mines be spammed and thrown directly at a player no the player actually has to step on them and then step off it to trigger it. How do you know sticky bombs are not OHK no one has said they are not. From the description on them it kind of implies they get blown up if not defused by a teammate within the allotted time period. 3 satchel charges can be thrown out at once who said that if all are detonated at once they cannotkill someone?
What purpose does a mine have if it does not have the capability of killing the person who stepped on it? With all the counter measures in place to off set it I see no reason for it not to be that is all I am saying.
Anybody can see a sniper, he's not a tiny little metal plate on the ground. You can avoid him even if you don't know he's there, and he has to actively try to kill you. Grenade's can't be "spammed", due to the cooldown period. An entire team using their grenades in coordinated attack is team work. Somebody has to be able to aim that sticky bomb for it to do damage. Satchel charges are rather large from what we've seen and if you choose to use all three you either have to keep them in sight to set them off separately, or know when the enemy arrives to actually get anybody. The mine has multiple advantages over all of these. Yes it has disadvantages, but that is the point of balance.
Mines are small, can be "fired and forgotten" if you want, do solid damage, and possibly knockdown opponents. If a guy steps on a mine (most will step off) and waits for a teammate you, who know where the mine was placed, now have a chance to kill two guys. You open fire on the guy and he steps off the mine and it's all over for him and his rescuer.
You plant a mine in a doorway, team races through and one guy sets it off, damaging everybody involved, your turret finishes them off. If it has knockdown it lays them all out as well.
There are so many possibilities with the mine, it's by far one of the most useful abilities assuming that it does decent damage. You seem to be afraid that it's useless the way it is because you can't think of a way to use it the way it is; the whole point of the way the game works is to expand your view and get you to see all the ways the toys we have can be used.