Much has been written about the relative greater success of PvE games over PvP games, or why in a game offering both a majority of players remains in safe areas like UO Trammel or EVE empire space. The theories range from impact PvP being inherently "niche" to the idea that PvP just "hasn't been done right yet". But while spending my holiday mostly away from computers, I came up with a different explanation: What if it is simply a question of time?
On the larger time scale, consider my holiday: When I'll be back to World of Warcraft after an absence of 3 weeks, I'll be able to continue as if I hadn't been away at all. The PvE content is still there like it was before I left, and my guild isn't so hardcore as to kick out players for a few weeks of absence. Now if I was playing a PvE game with lots of politics, territorial conquest, and warfare between alliances, several weeks of absence would be a lot more noticeable. A lot of things can happen during 3 weeks in an impact PvP game, and the more you are involved, lets say as leader of an alliance, the harder would it be to just take 3 weeks off. You'd probably find your position usurped by another player, and the situation completely changed.
On the smaller time scale, PvE content is easier to consume in shorter sessions. If you have just half an hour to play, you can do a heroic dungeon in World of Warcraft nowadays. But setting up a big PvP battle takes a lot longer. If you want to play politics in an impact PvP game, you'd better be online a lot. And games of territorial conquest become somewhat silly when everbody logs off after the enemy territory is taken, allowing the enemy to take it back a short time later.
While the press usually reports about the extremes, the players who play 16+ hours a day, the average player is estimated to spend just 20 hours a week online, and casual players might just be playing an hour per day, and not every day. That works fine in the pseudo-static environment of PvE games, but playing little and on an unpredictable schedule is certainly a huge disadvantage for PvP. So maybe the smaller number of PvP players can simply be explained by the average player not having the time to really get involved enough in a PvP game to really make the desired impact.