The game loop in PVP has a lot more diversity in builds than in PVE where players only follow meta and same sets every patch with some minor changes.

Of course you can't find more people than 7 groups of couple hundreds of people. First of all, it's the lag that keeps people away from Cyro. If in theory, 70 groups were playing, you wouldn't even be able to move or do anything, lag would be either so insane or servers wouldn't even work. Personally, when I see the campaign is full I prefer to not even join it because I know it won't be playable for me. It often takes to press the skill up to 10 times for it to work in a full campaign. I believe many others also don't play Cyro for that particular reason.

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Then it's also builds. A lot of people just don't like to be killed often while also can't find a strong build to play with. Most of the time you need to theory craft to find something good and strong, and viable while often youtube builds suck. Theory crafting is also very expensive.

Then it's balance which is not good. Stamina Wardens and Stamina Necros are bis in PVP now. There are OP builds that can be used on other classes but Necros and Wardens utilize them the best and for a good reason - they are behind the paywall. Just like Malacath is a bis item for most builds and it's also behind the paywall. This is another thing that turns people off from playing pvp.

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In the end, it's ZOS who made PVP less popular. Insane lags, imbalance, paywall, no new content - are the main reasons.