I believe this was just a money decision. You really can't run all those fancy tracking ads and other engagement/conversion tricks within RSS readers, or if you did, people would just switch to readers that didn't support ads and tracking. RSS was, mostly, just content that you could format and read as you like and that's a pretty big threat to operations like Google who rely so much on ads and tracking. Reading mode in browsers is the middle ground that sorta works. We still get presented with ads and tracking but get the option to view things in a sane format. Of course, we don't get the neat all-in-one aspect we had with RSS. We still, somehow, are using the bookmark system for visiting and consuming the web.