The same reason some restaurants focus on serving meat while others focus on vegan meals and others serve everything in between.
Optionality is good, specially when it’s basically free.
Discussion
In this example restaurants are more like npubs, not relays. If you want to parable relays, you should say city districts that only allow restaurants that serve only meat 😉.
No, npubs are guests inside the restaurants if you want to take it there.
A similar analogy was used in some podcast now that you mention, I think it was on TGFN, where they use the analogy of relays as houses
