Pvp in the legacy servers is so expensive that only the insane few left behind can afford to pvp anymore. So they left.
maybe the merge (along with the ongoing glacial upgrade IM sales) Trinity release will bring some back, but I suspect those will leave again when they see what it costs to compete here anymore. I jsut finished dropping $250 here and am nowhere near being geared for pvp (or pve) here on a lvl 115+10 character. I would be 1-shotted by just about any full-geared player in Ran. I doubt dropping in another $200 would get me anywhere near geared enough.