Firstly, congrats to "team chris thormann" in Blizzard's North American WoW Arena finals.
No, I'm not actually cheering the hunter on the team for once - it's the Aussie warlock I was supporting; even though they only won against one team at the event (and that team managed to defeat them later), managing to get to the top of the Arena board in the first place with Aussie latency is a bloody impressive feat.
Secondly, trying to excel at, and get Achievements in, PvP takes so much more individual effort than any PvE Achievements.
Sure, you can argue that it's difficult finding halfway-competent people to run a raid in the first place, but really... if you're willing to wait until everyone is geared up (or at level 85 in Cataclysm, for example), you'll finish all the PvE Achievements eventually.
Besides, if you're willing to spend a huge chunk of gold, usually there's at least one guild on your server that will run you through raids and Achievements for a price.
For PvP, on the other hand... if you suck, you'll never be able to get an Arena title no matter how many games you play... and even if your entire battleground team is communicating in Vent, the other faction can still pwn you if they're quicker.
(Props to that team of Horde, by the way... we didn't have any AFKers, only 1-2 PuGs, and a group of about 5-6 Horde - possibly an elite Arena team, but still - managed to roll us and win 1600-1590 in AB)