I think it's because, at this point in time, unless you are totally camped in there is no real reason to leave your station to fight invaders. You can sit pretty, and the only loss you have is that you're not able to farm or mine during that time. However, lost income isn't that high on people's lists of concerns as they play this as a game, and most of them can just go do something else in the meantime.

However, people become very attached to things they have made. That awesome refinery you planted down next to your favorite epi spot is a lot more easy to justify defending than an equal value of time that could be spent mining, simply because the structure acts as a focal point for emotion, both in terms of the joy that comes from destroying it and the loss you feel when one is destroyed. This sort of emotional nexus generates PvP, and therefor, content.

Just my 2 bits.