Both solid games. ESO uses (platform based) megaservers; FFXIV has introduced cross platform. Both help prevent "old fashioned" server population issues.
The other factor I suppose is that ESO doesn't need a monthly sub being b2p with paid dlc - the current target being one drop in each quarter. Not suggesting this matter to you but it probably impacts how people play. Probably because as mentioned the game uses megaservers. The significance however is that people are - probably - more likely to "stick around" even if they only play a few hours a week whilst others will "leave" and then "come back" for the dlc.
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But ESOs pvp was fun when played as a larger coordinated group (guild in teamspeak).
The other factor I suppose is that ESO doesn't need a monthly sub being b2p with paid dlc - the current target being one drop in each quarter. Not suggesting this matter to you but it probably impacts how people play. Probably because as mentioned the game uses megaservers. The significance however is that people are - probably - more likely to "stick around" even if they only play a few hours a week whilst others will "leave" and then "come back" for the dlc.
Beyond that maybe try some feeds etc.