Why do people always say that?
I do think that more PvP than PvE players have left, but definitely not most of them. I lost so many friends over the past years, none of those were active in PvP. I wasn't even active in PvP myself for the last 3 years or so, when I left in october.
Because that is fact.
I left Tera more than 3 years ago. And I did exactly because they killed PvP. First by "balance"patches" then by absurdal classes: reaper and gunner and then completly removed PvP option from PvP servers. The earlier version of Tera is, was better. And I realy would like to play old version (I mean 2012 or even earlier version - Our beta versions was KR 58 lvl cap patch and it was great) with outlaw system from 11 lvl.
I even installed the game when Valkyria class was released. I left on lvl 12 when I was unable to take outlaw quest on so called PvP server and noticed in patch notes PvP was taken away from PvP server (LOL? WTF?!). Without open PvP the game have no sense, no metter how combat system is good.