Here's some discussion about it

https://github.com/nostr-protocol/nips/pull/1051

Basically for it to work we need to change all clients, relays, services, anything that uses nostr

Its a pain to code

Without it being implemented in all clients and relays it will never work reliably

It will make relays slower

Clients can't use it without a relay supporting it

Read the entire conversation in the nips repo it will explain better than me, I really suck at explaining things