Well, one of the reasons.
First, if you like Okular, you're stuck if you want to run on any other platform. It doesn't work on mobile, it doesn't work on the web, doesn't work on Linux, MacOS or Windows [1]
Additionally, there were pretty severe bugs when I used it and there were 1-2 long outstanding bugs regarding annotations that weren't fixed for nearly a decade.
The main issue with PDF annotations also applies. If you use PDF annotations it means mutating the PDF which has all sorts of problems.
I've had issues where you're working on Okular or Preview or Acrobat and then try to use another app and your annotations simply don't work or the PDF is corrupted in some weird way.
Having an immutable PDF solves that problem which is the avenue I went with in Polar.
Polar supports Web + Linux + Windows + MacOS. It also supports annotation without mutating the PDF and it's VERY hackable since it's just based on Typescript - which is one of the features I wanted.
We're very close from having mobile too and just need to clean up some CSS.
Anyway.. would LOVE your feedback if you play with Polar and find it doesn't perfectly nail your use case.
I'm actively developing it now and I'm pushing 1-2 releases per week.
1. My understanding is that while it DOES work on these platforms - it doesn't work WELL on them. Last time I checked there was no download for Windows or MacOS and you had to install the full KDE for these platforms.