PvP related gamemodes are fun, and enjoyable for those players who do pvp, such as myself, but, under the current circumstances it just isn't necessary for a more survival based server. We already have Kit PvP, which is entirely dedicated to pvp. Which alone is costing alot of resources for Manacube to keep up and running, and is barely gaining any players in return. Overall, PvP just isn't that massively popular on the sever, that requires an entire realm dedicated to it.
Sometimes, staff may opt to hold a event in /duel that is somewhat like UHC, more like hunger games though. But /lps (last player standing) is also a Pvp related event.
When considering these factors, the key component are the resources to create the realm going to be, and if they'll make any money at all form it. If they were going to add a UHC gamemode, they'll need to pay for a server subscription, pay for GB to store it on a console, hire new staff, hire a accountant etc. Like most servers, Manacube is trying to make money, and provide an enjoyable experience for all involved. If the majority of players wanted it, they might take it into consideration, but one player that asks for it, over 1000 active isn't going to change anything, since they can't just run a server for you, for free.