Because other than main city general chat *cough /ignore cough* you don't have a smack talk style interaction with opposing factions. The reason I went with a pvp server, is I wanted the occasional confrontation to test my mettle. Most times it comes down to who can call in more 60's, but by and large you get some good fights. Or you can get rid of people trying to be annoying.
Honestly the PvP honor system needs to be retooled. I was mulling it around, and came up with a couple of ideas. First, players should have three PvP states. Unflagged, Flagged, and Active Flagged. The only main difference is that to be active flagged you have to "Actively" be attacking opposition players. If you are outside of the level range (say 9 above your target) you will start to incur honor penalties for killing "less than honorable" targets. Like the civilian penalty but is a phased approach. So while you fight people your own range or higher, you are become active-flagged. Once you start combat against a non-active flagged opponent below you range, you get a certain amount of time where honor does not get credited. Along with if you kill enough non-actives below your level you lose honor.
I think something like that would promote people from fighting within their own level ranges, but allow players to help others if they are engaged in combat with the opposing faction. It would also help to stem the "ganking" higher level players tend to inflict to those much lower than they are. For me I could care less about killing lowbies, though some people thrive on it. In the grand scheme of things, from a war perspective why would a "General" (IE Level 60) waste time killing a Footman (IE Level 10)? It should be discouraged to waste such resources on trivial targets. The grunts should be training to fight the footman or larger targets. Though it also allows the higher levels to deal with aggresive opposing faction players, without incurring a penalty.
Edit: Also what are you plans for WoW endgame? Raids for gear or PvP? I'm focusing on PvP just because after a while I can get the best gear from doing PvP rather than doing raids. I don't understand how the focus is always on PvE. Sure you get great items but there is a huge time investment into something that isn't guaranteed. While you can accomplish the same thing, but have a better benchmark to weigh in against for getting particular PvP set pieces. I mean, what do you do after you've collect all the epic gear you want? PvP... ebay...roll an alt? Like I've said, I can kill level 60's at 54. I can only imagine how I'll be able to pummel folks once I start to get the PvP sets. Let alone getting GM stuff.
The thing is nobody is paying your subscription but you. Why you are forced into instances and long raids is beyond me... It's about having fun. IE finding something within the game that fufills that function, or moving to a new game.
Edit2: They also need something like how in AV when you "kill" someone, you can loot the body and turn in materials for quest rewards and faction bonuses. As it stands now outside of the BGs, PvP is more of a gank or be ganked thing with little to gain from being on either side. Just like how honor is awarded, you should get more "PvP materials" from higher level and even more from higher ranked people. There needs to be a reward outside of the BGs. As it is, outside the BGs PvP isn't constant so the rewards don't seem they would be outrageous.