1. Goodreads has a LOT of users. It's the most extensive source of feedback on all my books. Lots of "wisdom of the crowd" that I can glean, looking at what people say. I'd rather have a kludgy site with 350 reviews than a UX masterpiece with only six.
2. Goodreads's huge user base means that reviews get noticed. This is crucial to keep the reviewing ecosystem going. When I put some energy into reviewing someone else's book that made an impact on me, I get a lively mix of upvotes and responses, which validates the time spent. Writing a crisp review on a minor site and getting no engagement is the worst user experience of all. Even if the official UX is beautiful.
3. Goodreads has pretty good tone control -- and that is not easy on any social site. People come to talk about books. Most threads don't get hijacked by MAGA/vs/woke. Anyone who overlooks this factor hasn't tried to operate a social site in the modern era.
4. Goodreads has the balance of power right between authors and readers. There are some things you can do as an author to drive engagement. But not a lot. You can't overwhelm the site with promo for a book that doesn't engage people. And Goodreads will stop you pretty quickly from flaming readers who give you one-star reviews.
All of these, I'll submit, are big, enduring advantages. They can't be swept away by a small new site with prettier UX or faster load times.