I'm not sure if it's prioritization so much as just Reader mode being easier because it doesn't need that much from the extension API. Adblockers need to do more browser-level things like intercept HTTP requests, while Reader mode can be done with just DOM/CSS manipulation.
Also, reader mode is the only way non-mobile websites are readable on my phone. I left chrome on my phone for it (now I use Firefox Focus as my primary mobile browser for the vastly better privacy, but very much miss that it seems to have no reader mode). If this has reader mode I am absolutely going to use it as my backup for when Focus breaks a site over Chrome.
When it came out I considered reader mode a killer feature, and coincidentally it also tends to remove the ads. Or at least the ones not embedded in an article, which is good enough for me as long as they remain stationary.
