Tunnel vision, got jumped on, "im doing my job trying to kill this guy so i will let others can do it", simply ignore, etc.

There are a lot of reasons so it is depending on your luck of which kind of teammates you got. I usually have a marker in my hotbar to point out important targets. Doesnt always work though, as sometimes people will just ignore it completely. There will be some people that even have the "why do I need to listen to you?" attitude as well.

Well, it is pretty much expected in a this kind of game as we are completely strangers, cooperation is very uncommon. Still, i think markers are better than spamming chat though, easier to see too.