Probably because sniping, which should be a niche role that only a select few pursue, is instead often much more popular than it should be because sniping is often much easier and more powerful than it should be. It isn't "normal" for a large portion of players to be snipers, and it usually ends up with a great deal of them being either A. completely useless, or 2. good at killing but no help at all with objectives (so still kinda useless, and often a real pain to play against), because a lot of them are in it for the wrong reasons. And people notice that because it's extremely annoying, from both ends.
What makes sniping a more "niche" role than any other defined role? The whole point of having a defined role is to be "that guy" for your team; you don't want a bunch of anybody, but it still happens with multiple roles.
When it comes down to numbers, there are far more players using the other weapons that are "completely useless" or "no help at objectives", because they're in it for their own entertainment, which is the same reason your aforementioned snipers do what they do.
"Often a real pain to play against" is more likely the reason why people go after snipers specifically, and not just the thousands of other players that end up serving no help to their team in various team games.
Anyways, point being that it's not the snipers that you're having the problem with "overrunning" the game, it's bad players/teammates, some of which happen to use the sniper rifle. The Title of the thread sets it up as an anti ninja/sniper thread (of which we have had many), regardless of the post that followed, I'm merely representing that "major" minority.