Well, goldmind's answer is pretty much exactly what I would've said, but I'll change a few things.
First, I haven't even tried Battlegrounds, and second, I have PvP'd in the past, like in the first year I started playing (on AnimaRO). I really only PvP'd with my friends though, just for fun. And it'd just be like, "Aw, you got me. Let's try again. =)" So really relaxed and actually fun. ...Then some random person would come up and kill us in an instant. Happened multiple times. It's like, whatever gets your dick hard, people. >:|
Plus, there's the whole gear aspect of PvP. Dispite being on this server for, what... 5 years? I'm still fairly poor. I do not make zeny very fast and I've somehow managed to get a DDPT MG, but that is the extent of my good equips (for the most part). This can be attributed to me being more of a casual RO gamer rather than a hardcore one. Just that dagger connot get me anywhere in PvP; I'll be dead in an instant. And why would dying instantly be fun? It isn't, so why bother?
Like, goldmind said, there's no reward for PvPing. And if there's one thing I hate in a video game that I love, it's spending hours or days on something, and not getting anything accomplished/having nothing to show for all the time I spent. (I'm also talking to you, Disgaea games. Especially you, Disgaea 3. >:| )
I absolutely love the PvM aspect of RO, so I'll just stick to that. Far less opportunities to rage quit.
For a shorter list...
- People take it way too seriously
- People tend to give obviously weaker people a hard time, going as far as attacking for no reason other than "lol noob im pro lookit me"
- Gear is too expensive
- It isn't rewarding
- And PvM is fun enough for me to not feel like I'm missing out on not PvPing