Actually, there are a lot of threads and there's another one that is almost 10 pages long.
Either way, there's a lot of different reasons. Mine is that I want to buy raid runs from an EU guild. Others have issues with legacies, want to switch servers to be with friends, want to get away from lag, want to fix their situation with RP (the whole Satele v. Star Forge thing) -- I've seen all sorts of different reasons.