Here are my reasons:
1) I have many alts with different professions that accomodate my main with what I need. I don't want to leave them behind nor do I want to spend $100 on transferring them.
2) I have two RL friends on this sever that literally just transferred yesterday to play together.
3) I honestly don't see a big issue with the server. We always end up with enough for a 25m, people who miss some days show up other days. We went from having to swap people every fight to having just enough each night. We turned down a lot of people last patch, and lost a lot this patch. I would say try agressive recruiting tactics, forums, trade chat, etc. (I am more than willing to help!) before transferring to another server that, despite having a high pop, might not have people who can do/want our raid times. It just seems like a huge risk to take on a problem that is experienced by WoW in general (25m dying because 10m gives the same rewards), not really our server (esp with server transfers being so easy now that off-server recruiting is very common). Who is to say we leave all our friends, alts, and memories behind just to end up in the same situation (or worse because not everyone will transfer so instead of having exactly 25 people at 8 we'll have 20 and no recruits)? Not to mention que times and lag outs for high pop servers are no fun.
I think that's all. I do see a lot of yes votes and I don't want to be the rock in the shoe, but these are my honest thoughts. This guild has been awesome and I wouldn't want it to go.