I am sorry, but the majority of the PvP community isn't like the PvE community.
Which means we don't have the mind set that more people means more fun, that PvE players tend to have with raids.
Honestly battlegrounds are fun in other MMO's but Arenas is where PvP becomes really fun imo as a PvP player, I mean a lot of MMO's skip on this feature when it is a good feature.
Now the counter arguments that I am most likely going to get.
It will get people out of Cyrodill.
Wrong, even though PvP players enjoy arenas the majority aka casuals tend to just stick in "battlegrounds or in ESO case Cyrodil" also you will need Cyrodil gear to actually go into Arenas.
If arena is implemented it will give gear and then people won't do Cyrodill.
You still need to go to Cyrodill, to get "basic" pvp gear, and how is it fair for PvP players that as the game progresses we will be forced to PvE since VR10 gear will soon become better then cyrodill gear as more and more adventure zones get released while Cyrodill content stays the same.
I am a PvE player and don't like Pvp content.
Well umm why are you even in this thread.
tl;dr: As more and more adventure zones will get added, soon a player that wants to be focused just on PvP will be forced to go PvE for the best gear to be competitive, so arenas are a Pvp solution (Instead of just updating cyrodil gear every time a new adventure zone gets added.(Which will anger PvE players since they will be like grrr we have to try so hard for our gear and pvp players just do the same thing they have been doing for there gear))