high sierra was one of apple's least-popular OSes (check this forum, for example). mojave is sort-of the improved, repaired version.
I know that HS had its issues, especially early on, which I waited a while to upgrade from 10.11 to 10.13, totally skipping 10.12. I think HS ended on a very stable version from my experience on 4 different Macs, all over 6 years old.
But, this forum is totally flooded with post with Mojave issues, especially with older hardware, HDD, and Fusion Drives. With HS not currently giving me issues, I will stick with that for a while, maybe upgrade to Mojave before 10.15 comes out, or just skipping it.
I play WoW, well I used to, and I am kind of getting back into it. I played from 2007 to 2016, and started playing again last fall. I was checking out the system requirements, and they have changed. I also was checking out if people was having issues on particular OS or HW.
WoW has been plagued with issues Mojave, so this has been one of the biggest reasons for me not to upgrade.
Besides, there really isn't any "must-have" features to have on Mojave to me.
So why upgrade?
It really depends on what hardware the user has, but everything I have read, Mojave may not be the best if you have a spinning drive or older HW.
For performance, my main Mac, a Late 2012 iMac is still running like a beast, so this isn't a reason for me to upgrade atm.
High Sierra is still getting security updates, so security isn't a reason for me to upgrade atm.
I currently don't have any compatibility issues with HS, so compatibility isn't a reason for me to upgrade atm.
those seem like pretty good reasons to upgrade.
They are good reason to upgrade, but if you can achieve these things on the current OS, they are also good reasons not to upgrade too.