The reasoning given is that maintaining a native version of uBlock Origin would be very costly, and so not worth the focus right now, especially considering that many of their users are already familiar with the extension.
I see. Is there any chance that we could use Brave's adblock? (probably not trivial because it's for Chromium)
I don't think it's trivial, as it is built on Chromium instead of Firefox. However, if you or we as a community wanted to experiment with adding a similar native adblock using the Brave version as a model, I don't think the maintainers would be opposed.