For a couple of reasons. Firstly, as everyone else says... bullies and douche nozzles. Going into a game is an act of enjoyment and escape for 98% of people, and its hard to escape or enjoy yourself when some little fusker is teabagging your freshly sniped corpse.
This. I'm a bit competitive when I play games for sure, but I HATE any kind of PVP in most games. Sure sometimes I'll have fun in an FPS on multi-player but it's few and far between, because there are people out there who get
obsessed with certain games and all they do is play the game 10 hours a day and it's not good enough for them to have a good match or a nice little competitive battle, they aren't happy until they crush the ever living hell out of other people 19 times. Those people suck the fun out of the game for me and unfortunately there are a LOT of people out there like that who will just go about killing, sniping, ambushing and using any kind of crooked tactic to gank other people who are out to have a good, casual time. I'm not saying they are in the majority, but I come across them way too often in PVP games. People like that, suck the fun out of the game because they will use any kind of broken method or exploit just to kill people, it's no longer about having a good battle they are just obsessed bullies who have more time to find ways to exploit the game than many of us "casual" types do. So joining a game that has no PVP or severely limited PVP (as in it's entirely voluntary and I can walk around and never once have to get attacked by another player) means that's several less pains in the foot I have to deal with during my game experience.
Except early on in the Beta or quickly after the Beta of Going Rogue, there was a fun little glitch I ran into a couple of times where I could fire on my own team once I entered a mission map. At first it was kind of confusing, in the middle of a pitched battle, I hit Tab to select the next enemy, immediately firing off some shots and I realize it's my own team mate. I point it out to my fellows and we all bugged it and continued about our day on the "honor system." Of course I couldn't resist and fired on a couple of my team mates, just to prod them a little and got justly rewarded with the entire rest of the team targeting me and taking me out very quickly and soundly. I think that happened a couple times, I'd not be able to resist just for the fun of it and they'd put me down like I'd talked about their mothers. But then that was all in good fun and not about just trying to Pwn everyone on my team.