Technically because most clients don't have that UI.

But practically, how would you choose? All relays are same from users' perspective, there needs to be a much richer UI and much more info provided by relays about what the relays' 'topics' are, what it's policies are, what is or isn't allowed, etc.

Besides, no relay is doing any real moderation, because there are no tools, and so even if someone wanted to run a topical relay, it would be hard to enforce the content policy that relay wants to have. Which makes choosing a relay somewhat meaningless, even if you could do that.