Because some people invest emotionally in their guild. He mentioned he'd been there for over a year. That's a long time. If / when he leaves he will quite possibly anger, hurt, or otherwise strain his relationships with friends he has in that guild. Sure he'll meet new people to bond with,but initially it will be awkward. I'm not saying it's impossible to stay friends with former guild mates when you leave, there is certainly a right and wrong way in which to leave. But from my experience, no matter how you try to ease the transition, and keep in touch, only a few of your friends will actually still be friendly at the level you were at before.

Guilds in game have a strange quality to them. In one way, it's like summer camp -- when you meet people at summer camp you are bonded with them, and spend everyday with them, doing summer camp activities. But remove the bond of summer camp, the glue if you will on the relationship, add in new friends, different locations, and suddenly people don't write as much as they say they will, and people drift off. Suddenly you don't get the inside jokes that develop through raiding. You are an outsider on the drama/loot dealings, etc.

Guilds are not quite like summer camp in that summer camp is brief and lasts only a few months. Not an entire year. They become for many people, the entire experience of what their wow game is like. Their guild -is- their game. Sure people have outside friends, but the bulk of the time you spend in game is with your guild.

Deep down the author knows he has 3 choices really.

Suck it up, feel left out, and deal with a situation which prevents him from raiding or feeling like part of the team.

Try and make his own raid group/find a weekly pug to participate in.

Or leave.

He probably already knows he needs to leave. He more than likely wrote to get the push he needed to do so.