Its easy to avoid mines and u can only have like 6 at a time so its hard to just cover the map, and alot in 1 place isn't that good unless your their. i mean if you dead or can't see the area doing damage isn't enough unless it happens more often. Snipe can do alot of damage but only really if you far away and still not as much as many other spells. If u could cover the map with mines that would make it better but u cann't alowing the effectiveness of mines to drop, and making it almost impossible to kill demigods with high health or fast regen. He just can't dish out 4000 damage fast enough without mines, but if he does use mines he'll prob die.
Mines were not only good at doing damage but at trapping you opponet, throw them in front of him when he's fleeing and he has to take the time to go around, and also very good at getting people away from mine but now u have to get way to close allowing them to hit u by the time u throw them.