I have an immense respect for the officers and raid leaders of my guild, because they have such a hard job whenever they try to put a raid together. You can't just take the first 10 people showing up or do a lottery; you need so and so many tanks, so and so many healers, this or that crowd control, this or that special class for some special encounter, and only when you have filled all those slots there are a few spots remaining for "random" dps classes. Apart from the classes, you often need to sort out raid IDs. And if the raid isn't trivial, you better check if people have the right talents and gear. And in the end there is inevitably some guy who is sulking because he didn't get invited.
Once the guild is past Karazhan, the next big organizational problem is how many of which raids to organize at what time. Keep farming Karazhan? Go for Zul'Aman or Gruul? Do an all out assault on Serpentshrine Cavern? Or some mix of all of this? Most likely there are people more and less advanced in your guild. The more advanced ones already have all Karazhan loot and can't stand the place any more, while the less advanced ones would need a couple more Karazhan runs before being ready for SSC. Juggling all of this is hard.
From the point of view of the guild member, the same problems appear, only from a different perspective. You have a raid ID from yesterday, but the people forming a raid today want to do a fresh start. Or the guild decided to go to a different raid dungeon than you would have preferred. Or you are of the wrong class, or wrong spec, or not geared up enough for the raid. Or the raid you want to go to is organized just on the one evening where you can't play. Et cetera, et cetera.
Now compare that to the alternative of doing battleground PvP. The epic rewards are of a similar quality, even use the same models in many cases. There is no raid ID, no class requirement, no spec requirement, no gear requirement, no time and date requirement. In the most extreme case there isn't even a requirement to actively participate, as the AFK leechers in Alterac Valley prove, although I'd always prefer to at least try to give my best. You can log on, jump into a battleground, play like you want, and even if the whole thing ends up in a complete failure you'll get a mark and some honor points. There are no repair costs, no costs for consumables, and nobody expects you to pay hundreds of gold for enchantments. And you don't need to pray for a lucky drop of the epic you want, you'll be able to get what you want by simply buying it for your marks and honor points.
I was discussing how to equip my warrior with a reader who sent me a couple of tips by e-mail. But he is a hardcore raider, and so he suggested I wait for patch 2.4 and buy the new sword for 150 badges of justice. Problem is I don't have 150 badges of justice on my warrior (he has 7, my raiding priest has 73). If I did the daily heroic for a month, or got into Karazhan raids every week for two months, I could gather 150 badges of justice. But of course by doing either 30 heroics or 8 Karazhan clears or a mix of both I would in all probability already wear lots of epics. The badges are nice to fill gaps, buy an epic for the one slot where by bad luck you never had an epic drop. But they aren't an alternative to PvE epics, they are a complement. If you are still in blue gear, your guild is raiding SSC or Tempest Keep, and you can't get a heroics group together on raid night, your best alternative is PvP. I'm not quite sure yet whether I really want to do PvP with my warrior, but I'm well aware that it would be the fastest way to equip him with epics.
And if even me, who hates PvP, thinks of it as the best way to gear up, you know why PvP "rulez" in WoW for the moment. It isn't that PvP gives "welfare epics" or that PvP rewards are too good or anything. If you can get into a series of Karazhan farm raids, like my priest did, you'll get a lot more epics in a lot less time, and those are really welfare, the welfare that your guilds gives you, not Blizzard. But to get the PvP epics you don't have to jump all the organizational hurdles and overcome all the constraints I listed at the start of my post. You can PvP whenever and however you want. Nobody can kick you out of a battleground group. And you don't owe anyone for the epics you receive at the end.
Imagine a WoW end game without any epics or rewards. Would be far less popular, I know, but it is just a thought experiment. If there were no epics, people would probably do the activity they like most, because entertainment would be the only reward. You would get some sort of "natural" distribution between soloing, 5-man groups, raiding, and PvP. Back to the real WoW as it is now it is easy to see how the rewards move the distribution away from the natural one. Lots of people do PvP not because they like competitive gameplay more than cooperative gameplay, but because PvP is the playstyle with which they still can get rewards without overcoming all those organizational problems of raids. Meanwhile heroics and raids are underpopulated relatively to the natural distribution, just because they are harder to set up.
And that is an aberration of bad game design. It is totally possible to imagine a game in which PvP is hard to organize, while PvE raids are of the jump-in-anytime variant. Being hard to organize is *not* an inherent feature of cooperative gameplay, being jump-in is *not* an inherent feature of competitive gameplay. If PvP was limited to premade groups, and only the winning group would get a reward, organization of PvP would become as difficult as organizing a raid is now. And if there were raid events for which you could queue up, get in anytime, have a reasonable chance to succeed even with that pickup raid group, and get a point reward regardless of whether you win or lose, a lot more people would be raiding. Having the better organized players in raids and the less organized players in PvP is just a design anomaly of World of Warcraft.