The servers for ESO are also quite a bit bigger than for WoW, which means a lot more players. There also doesn't need to be a defined goal for level 50s to gank and grief lower level players (PvP flagging does take care of this though). There were an absurd number of players in Cryodiil, more people assaulting one base than I've seen on one of the full WoW servers in the main city. There were also at least 2 main forces of one army of about the same size. This doesn't even begin to include the other 2 armies.
If these numbers of people were unleashed on other faction territories, they would likely storm a main city and constantly grief lower level players by wrecking their quest progression by killing the NPCs.