I think there are lots of reasons. Here are a few…

- It’s complex to implement and requires a hardened ingester (sourcing notes from relays you didn’t select may be malicious)

- Performance and bandwidth penalties. Significant connection overhead increase.

- Clients who support NIP-65 often just copy kind 3 style 20 read/write relays in their list instead of the 2-4 the spec recommends

- Changes are still being made to NIP-65